I definately think going with a permanent donation button is a great way to start. If you put up a note in the news section when the hosting bill ocmes up a note in the news (Like torstan said) people will donate at least a little bit.
With a paid subscription you need to offer something fo rit. I think this site is great I check it basically daily (eventhough I rarely post (A happy Lurker I am)) the only thing the site offers of value to me s the forums, and the tutorials. I personally will never pay just to be able to use a message board. As for the tutorials, I believe it is one of the greatest draws to get people here initially, however if you charge for them you won't get many new people to the site.
The cafe press type merchandise is a good idea but I don't think you would get much out of it.
Another idea, though I don't know how low overhead or popular it would be (probably not to popular) , is maybe create somesort of map pack. Maybe once once a year, a pack with all the winning maps from the monthly challenges in a single download for one or two dollars (would obviously require winners permission or new rules to be inserted into the challenges allowing you to do this). Along with this idea maybe something can be worked out with gameprinter for this map pack to be professionally printed and shipped.