Here's a novel idea, why not use Kickstarter.com to fund module development?
For example (because we can never have to many!) a new Image Slideshow module. The base price would be say $1000 to kick start the development process.
Kickstarter allows you to have up to 3 rewards (I think it's 3) you could use this to tie in potential users and customers. Here's an example scenario:
Reward 1 - (Quantity 4) - $25 - You get 1 feature of your choice added
Reward 2 - (Quantity 4) - $50 - You get the module installed and implemented on your site
Reward 3 - (Quantity 5) - $100 - You get my first born and implemenation plus 2 hours support.
Assuming you get enough committers thats almost all $800 is covered by *potential* customers. Not a bad idea huh?
The reason I like Kickstarter - https://www.kickstarter.com/projects/923846239/rosetta-junius-split-record - Commitment complete within 3 hours of it going live and way more commited since, also they are a damn good band (shame about the UK tour though).