Rates used to be: $3.00 for 10 lines or less, $5.00 for 11 to 50 lines, and $0.10 per line rounded to the nearest dollar for poems over 50 lines.
Rates are now: $0.03 per word, minimum payment of $3.00 per poem.
Someone mentioned that Horror Writers of America defined "professional rate" is $0.25 per line with a minimum payment of $5.00.
Duotrope's Digest defines rates as:
Token: less than $5 per poem
Semi-pro: $5-49 per poem
Pro: $50+ per poem
Also worth mentioning is that many semi-pro and pro markets do not pay poetry on a flat fee or per-word basis. They pay per line, which I think is a good idea.