Drilling out a bolt

[brainbug007]

So I tried to take the exhaust off my bike at the weekend to respray it with some VHT paint as it was starting to rust. Unfortunately the bolts used to hold it on where mega cheap/weak and one of them has snapped off in the head which is aluminium I tried drilling it out without much sucess as my hss bit just don't seem to really want to drill into the bolt. Seems to just make a small circular pit in the bolt and not really go any further. Any suggestions as to where I'm going wrong? Also if I take it to a dealer how much are they likely to charge to fix it??