Hi I have worked on executive compensation in multiple companies and I am presenting information based on my recent work in this area.
COO salaries in the US will range from USD 105K to USD 268K depending upon experience, location & skills being hired. This will include base pay + 401(k) benefits + Performance incentive/bonus. If you are offering equity then thats an excellent negotiation tool.
You would have to first decide what compensation positioning you want to take or have already taken wrt your existing talent at developer level. This can determine a pay range - then look at your budget & discuss with your recruitment partner if you can identify talent at that cost. Keep a range say from 75% to 90% of the compensation ratio as your target cost for a role like COO.
Take a look at tools like Robert Half - rht.com.
I am happy to help you with the details in case you want to do a more detailed analysis/study.