It works like this: If minnowbooster's power is 100%, a regular minnowbooster vote will be worth about 3 times what you paid. A stealth minnowbooster vote would be worth about 2 times what you paid at 100% Votepower.
Check minnowbooster.net for more information. It also states there the worth the votes will be per SBD you send to minnowbooster. The stealth vote will yield you a vote worth 1/3th less than a regular vote yields you.