I've been working on this a while and I see the conversation is moving on, so I'm going to post what I've got so far. This is giving an example of 4e rules in this regard, in case you are able to come up with a conversion.
So, in 4e I know that, for example, an item that is level 11-15 can grant a +3 bonus to a particular skill (like Intimidate), or a range of skills used in a particular way (say, checks related to social interactions where social class is relevant). And the higher level the item is in that range, the more additional perks (like applying to a second or third skill, or non-skill related abilities) the item can have. And I know the cost of such items (a level 11 item is 9,000 gp, and a level 15 item is 25,000 gp, for example), and the way rarity interacts with the game (only common items can be purchased or made by the player in ordinary circumstances). Moreover, some items require upkeep or they fade/lose their benefit - usually an alternative reward fades every five or ten levels, depending on the type of reward.
So I could, for instance, classify a title of nobility as an uncommon or rare reward (it can't be bought, it must be bestowed by the sovereign, and that its benefit fades over time if you don't put money into it - in dressing appropriately, maintaining social/political contacts, maintaining property, etc. So I could say the title of a Marcher Lord is an 18th level item that grants a +3 bonus to social skill checks where rank is a relevant consideration, and that it also grants you the right to clear lands in the borderlands and build a castle (allowing you to treat two "uncommon" items as if they were "common" items); to attract followers and peasants once you have your lands and castle; and to receive an additional bonus to social checks once you have lands, castle and followers.
Now your lands don't produce cash (unless you are lucky enough to score a gold mine), so you can't buy things with what they produce, at least not directly. But you can use the in-kind production to support your castle, household and troops (with horses and equipment), and to raise troops, or to reinvest in the estate to improve it - and also produce clothing and other materials to support a certain standard of living for the noble and their family.
Units of troops would be treated in many ways like mounts. That is, there is a predictable relationship between the level of the mount and the cost of the mount, so a given value of estate ought to be able to produce a unit that, as a group, acts as a creature (swarm) of a particular level. Also, in 4e mounts don't use their own actions in battle, they need to be directed using the PC's actions, so a unit would need a PC or NPC officer to tell it what to do in most circumstances.
The thing that would take a little thought, and trial and error, is to determine how many troops and NPC officers an estate could generate and support. A modest estate would support some vassal knights and their retainers (a handful of cavalry, light infantry and noncombatants), warhorses for the cavalry, riding horses for other essential personnel (anyone you don't want to leave behind if you end up in a hurry, which might include infantry if you can afford it), and riding horse remounts for everyone who has a horse (possibly two for cavalry, because you want the warhorse to stay fresh). I was listening to a podcast the other day which mentioned a real life example of a modest lord whose retainers included 50 men and 95 horses (and I think he mentioned that the vassal knights' retainers would have been on top of that). Plus a militia for short term local actions that might raise one soldier for every 25 people, and a general defensive militia for emergencies that might be one soldier for every 5 people.
I note that in 4e the cost of all those troops is far more than what could be raised through adventuring, so it would have to be the case that it all comes from estate earnings in kind. In theory you would need an impediment to spending that income on magic items instead, so you might limit the ability to convert grain to cash, for example. In practice, in 4e there are a limited number of items available for purchase or creation by PCs, so they aren't likely to break the game (and I note in the Eberron setting while magic items are widely available, that is limited to low level magic items).
Fortifications provide protection for troops, of course, and discourage raiding (since your cavalry can sally forth to catch raiders. The more expensive, the bigger the garrison it can hold, and the more protection it can give to them, and the more other features it might have. But instead of pricing them using a stronghold guide, you price them like items with similar capabilities. So a pallisade fort might cost 25,000 (level 15), house the lord's household and 25 troops and officers (I'm making that one up, it would require some thought to systemize), and provide superior cover to defenders and a +3 defensive bonus (if the PCs aren't running the siege and you want to resolve an attack with a single die roll), as well as a +3 bonus to ... whatever skill checks seem appropriate to a castle. I'm not publishing it, so I could make ad hoc decisions as I go. Certainty intimidating raiders to convince them not to raid, but perhaps also to perceive hostile activity on the estate (due to watchtowers and scouting parties). But I know the size of the bonus when I need one. And a large stone castle would be more expensive (call a keep with a walled bailey and a gatehouse 85,000 gp, level 18) but house more troops, give +4 bonuses, and maybe contain a library (+2 bonus to most knowledge checks) or other special rooms (some of these already exist in the rules, like an Alchemist's Workshop (level 8, 3,400 gp, lets you make higher level alchemical items than you normally could).