slominer,
“How much is considered normal to tip the barber for a haircut? Is it more for a haircut plus trim?”
Assuming you like the haircut/trim you got, a normal tip I think is around 20%. Tip generously if you like your barber and intend to be a repeat customer.
“What is the typical price range frum barbers charge for a haircut? And for a haircut plus a beard trim?”
I don’t have a frum barber, but my barber charges $15 for a haircut, and an additional $4 for a beard trim. I tip $4 to $5, to bring the grand total to $23 or $24. This price is quite reasonable, even low for the area, and his work is top notch – with both attention to detail and about as fast as I’ve seen.
“Is using non-frum/non-Jewish barbers inappropriate? Are their prices generally more/less than frum barbers?”
As long as the barber can understand and follow your directions on how to cut your hair, I do not see why it would be inappropriate to have a non-Jewish barber.