By far the cheapest I’ve found is to go through McMenamins...might be good for a grad party...worked for a friends wedding for me. Their beers worked well for the non-beer-nerd party.
From their site....
How much will a keg of McMenamins beer cost me?
A keg of McMenamins beer costs $85 (this is a seasonal discount from our normal $105 price). All kegs require a $50 refundable keg shell deposit. Tapping equipment requires a $100 deposit, $90 of which is refundable upon the tap’s return. McMenamins can only accept cash or credit cards for keg sales and deposits. Please note: keg shells and rental taps must be returned to the brewery of sale to receive a deposit refund. A sales receipt must also accompany the shell and tap to receive the refund. A refund check will be made payable only to the purchaser of the keg.