I disagree that money is the most important factor, especially in recruiting designers. Having worked in Silicon Valley the past 5 years and with both Visual & UX design recruitment a speciality, I've sourced, screened, and closed (and missed) on my fair share of designers. They tend to be looking for 2 primary pieces for their projects:

1) Interesting or exciting project, which allows them to use things they've recently learned or technologies they've picked up. They want to feel like they're staying current and moving their career forward. Since it's a project and likely not long-term, they'd like to feel that it will be a boost for their portfolios.

2) Creative freedom and be viewed as problem solvers. Designers love feedback when they're missing the objective of solving the problem (and you should ensure you're hiring designers who view themselves as problem solvers). What they don't care for is a non-designer micro-managing their use of a certain font or color.

Money, work environment (likely telecommute in your case), and other benefits are likely tier 2 motivators.

If you want to be a top destination to attract them, give them the confidence they'll learn and be energized by this client project and you're going to treat them like a problem solver.

I've hired designers for Google as well as a dozen or so startups. I've also hired about 20 people myself via upwork/odesk for my own consulting business and understand the freelance community, if you'd like to discuss more feel free to schedule a call. Good luck!