I tested that once and if you click on the little down arrow next to the dollar amount something like this pops up:

Pending Payout $0.92
(0.46 SBD, 0.13 SP)
in 5 days
Promotion Cost $31.00

The "promotion cost" is how much the user had to pay in SBD to secure the #1 spot on (in this instance) promoted/steemit. As you go down the list, the amounts get lower and lower. When I tested it, I put a few SBD in it and got a small burst of votes/follows but I did not secure the top spot. I have seen users spend up to 50 SBD to reach the top but when I looked at their stats (steemd.com/@username) I didn't notice a response worth the amount they spent. YMMV.