Yep, the amount is up to you.  If you want to 
just deposit $1.00 thats fine.  Whatever you're 
comfortable with. 

If you make a cash deposit (like paypal) you 
multiply your deposit by 5...and thats how much 
your limit will be roughly.  By current 
calculations, a $5.00 deposit would get you a 
2,750 yen bidding limit.

If you have any questions just drop me an email 
or post on the SMJ forums.

-Nick (owner of SMJ)