There is an extra fee for having a reserve, but it is refunded if the item sells. The reserve fee depends on the amount of the reserve:
Reserve Price Fee
$0.01 - $49.99 $1.00
$50.00 - $199.99 $2.00
$200.00 and up 1% of Reserve Price (up to $100)

More here:
http://pages.ebay.com/help/sell/fees.html

I usually won't participate in a reserve auction, unless the item is great.

Robert