Advice please ABS

I have a 2006 pontiac Montana, it's been running great, it has 91000 miles on it and recently my ABS light has come on, it's happened twice in the last week and I'm wondering what the most likely cause (required service) it might be, and what the cost is likely to be, I just want to have a little knowledge before i bring it to the shop, thanks for any help or advice.

There is no way to know, without at least reading what the ABS code is.....a scan tool capable of reading ABS codes is required....