Buying doujinshi can be quite expensive when you bid on already inflated auctions, where you'll still have to pay for shipping, service fees, and wire transfer fees through a deputy service.
While auctions are great for buying truly rare items, you should always check Japanese online stores like Messe-Sanoh, Toranoana, Melonbooks, and Mandarake before placing a bid. Not only can you get an idea of the "retail" value of a doujinshi, you can also see if they happen to have it in stock. All of the shops mentioned except for Mandarake, only ship internally (to Japan addresses). To work around this, deputy services--companies that buy these items on your behalf for a service charge--are plentiful.