I think it's hard to estimate the effort for bug fixes before you've diagnosed the problem, and diagnosis is often the lion's share of the time spent.
If your bug volume is fairly consistent, I would just let it "come out in the wash" against velocity. This is what I usually do for production defects that impact a team's iteration goals.
If you realize mid-iteration that you're falling behind (e.g. you see a burn-up chart that's not looking like it will intersect with your scope line by end-of-iteration) due to bug issues, then you can adapt scope (drop out the lowest priority story) to accommodate the extra work.
