I'm what most people would define as "goth" or "metalhead", and most of the time I wear black jeans and long sleeve tops that have skulls or dragons on them. Most people think I found them at Hot Topic and paid them a small fortune. Well, think again. They are from Wal-Mart or Zellers (a Canadian store, a bit smaller than Wal-Mart). I found them in the little boys' aisle. Once, my fiance joked that I should try on kids' clothes to see if they fitted, since they were less expensive than adult clothes. Since Hello Kitty, Bratz and Britney are really not my cup of tea, I checked out the boys' section and was surprised to find so many beautiful tops, all under 15$. My favorite are a dark grey one with red skull motifs, and a black one with a Matrix-style creature. Of course, if I weren't petite and flat-chested, I couldn't wear these. But I've seen many interesting goth-ish longsleeves and hoodies in the men section as well, they are just bit more expensive (around 20 bucks).
Another related tip from a pregnant girl at my job. As a punk, she wanted to stay away from ugly and expensive pastel maternity dresses. So she bought several pairs of XL pajama pants for men, and several XL black tops, which looked nice with the plaid pants. If the legs are too long, the bottom may be rolled up or hemmed.