cheap but effective: get their music played on a local/regional college radio station if at all possible.
And if you're willing to invest a little time and money into helping promote the show street team style, then you can always print off a good amount of flyers to leave at places you think a metal crowd would frequent. I usually hit up all the tattoo shops in the area, as well as bike repair shops, music stores, cd/record stores, comics shops, electronic gaming stores, skate/surf shops, college commons, etc. I've been learning more and more places to go.
Make sure the flyer is colorful and eye-catching. In this case, use a great current promo shot of the band. Try to print a fair amount of them as 4 images to an 8X10 page. This would give you four times the amount of flyers to spread around. Plus business owners do not usually want big sheets of paper cluttering up their counter tops.
Always approach the business owner and ask permission to leave the flyers. A good tactic is to ask them for a business card, flyer, or leaflet in exchange for leaving a small stack of flyers with them, and offer to leave their card/flyer on a merch table at the venue the night of the show. I've not had a business owner turn me down yet.
Always go back within two days to check on the status of your flyers. Never leave more than 20-25 to start with. Because even if the business owner is cold-hearted and throws your flyers in the trash as soon as you leave out, you won't be out too much money. Typically all 20-25 flyers will not be taken within two days. And if a
few are missing, then great! - leave a few more. If you check back a couple of days before the show and most or all of your flyers still remain, then it's a good idea not to waste your time or money returning to leave flyers there again.
One more item of interest I've found useful, but not sure the effect yet... print about a hundred business cards stating the pertinent info about the show and always have 10-20 of them on you or in your car. You never know who you'll meet on the street or at a bar or coffee shop who expresses an interest in your Nightwish shirt/hoodie/cd.

Besides, people tend to put a business card in their wallet or purse, whereas a flyer ends up in the trash 75-90% of the time.
sorry I kind of rambled on there, but these are some tips I've learned along the way. I hope it helps.