The PC community has been spoiled for decades - most people want and expect these great changes to occur within the games we love and content creators do it for the love of the challenge and a way for them to make a difference in a game they love playing. Donate buttons are nice but let's face it - they are rarely used and those who say they would, most never do. It is the sad reality of the community and inertia people have when they are used too (and think) things should be free. You (like others) have made some real game changing functionality to the game that the developers are unwilling or unable to do. I think it is perfectly acceptable to ask for a fee or contribution when there are major fundamental changes being made to the way the game is played. It is incentive for you to continue, it provides you a little "something, something" for your time & effort and more importantly it allows the developers to see what is important and wanted by gamers. People $%&^! about paying $3 or $4 but something that is adding real changes to the game - people will pay if it matters to them. I know I would.
Hardcore gamers (and those like myself who are hardcore city simulator fans) - will spend the money if we can get our ultimate goal - a modern day SC4 with all of the important mods included that allows us to micro manage the hell out of a city and every aspect of it. I can remember agonizing for hours while playing SC4 when my spreadsheets show commute times longer than 30 minutes or crime affected a certain block or area of my map. I may not be the norm but I love that kind of game play.
If you are passionate about the game you play and the MOD changes help you achieve your game goal, you will pay for it if the developers won't do it. Casual gamers and those who think spending $5 is the end of the world - they will never pay for anything no matter how good it is unless popular opinion and the buzz says it is the "must have" mod. Go for it..you deserve it and thanks for what you do for the community.