I usually try to find a seamstress who works out of their home but it can sometimes be difficult to find. I start by asking at my church but you can ask any acquaintances. I once found one by asking the clerk in the Men's Dept at my local JC Penney.
That being said, I usually pay no more than $8 to hem slacks or jeans. I've been charged as much as $12-15 by having it done at a dry cleaners store. The waistline can be a little more difficult because garments can be contructed differently and some ways are entail more work for the seamstress to take apart and adjust than others. $15-20 should cover it. HOWEVER, it all depends on whether the manufacturer left any excess material in the waist for the seamstress to work with; if not, it can't be done.
I hope this helps. =)