My last company had a technical cofounder who by all means was highly sought after and managed a team of 15-20 other developers in their primary dev consulting company meaning not only did they rock technically but also had biz acumen.
That individual worked FOR FREE for 3 years 25 hrs/week before collecting any money.
Don't ever think you have to pay a developer some market salary (or any salary) upfront. If the idea + company + team is legit, they will jump at the chance to be involved.