Nation funding is not the easiest thing, you only have 1 tax which is town tax and if you don't have a lot towns this number can be pretty high, generally though when you have more people funding something, they don't need to give as much money; So what if you were to add a resident tax. I know my nation (the Taurus realms) has 14 towns and about 410 people squeezed into it. If I had to tax every town, they would pay 357.14 a day, If i taxed every resident it would total to just 12 meebles a day. This would be added on to the res tax when you type /t like so:
Taxes:20(+12 bonus)
When towns join they must make their rules include this tax but it would be the king's responsibility to make sure that it is posted in every town.