The Landlord feat might work ...
... but it has issues of its own (being 3.0e, requiring use of a book no longer in print, etc). You can hand wave a lot of it, however.
Landlord [special] (source: Stronghold Builder's Guide
Prereq: PC must have 9+ levels.
Benefits: The PC has a small allowance of virtual funds that can only be used to build or expand upon a stronghold. These funds are virtual - they cannot be spent on anything else. Also, if you spend your own funds beyond the virtual allotment, part of your funds are matched (about 1/2 to 1/6).
Example: A 12th level PC takes the feat, gaining 100k gp worth of funds to build a stronghold. Beyond this, for every 100k they spend of their own money, they gain a virtual addition of 25k. Upon attaining level 13, they gain an additional 150k virtual gp to spend on their stronghold - and for every 150k they spend of thier own funds (beyond the initial 150k), they gain 50k additional virtual funds to spend on their stronghold.
Personally, I think this works rather well for a nobility type feat, in that most nobles gained and held their title - and gained whatever wealth they had - only through ownership of land (and the use of those resources - natural and human - upon said land).
The details, however, should perhaps be considered a bit iffy. The actual level of virtual funds, for example, could be modified based on the campaign world, the level of rank, the location of the lands, etc. Also, without the Stronghold Builder's Guide (whose prices were rather suspect / odd in many respects) the virtual funds mean little as you do not know the cost of the items you need to buy with said funds.
This is a WotC source, but there are also quite a few good books on such from 3pp to consider.