We can't give you an exact price without knowing exactly what you would like done, but here are some average prices for roughly what you can expect to pay.
Hemming a dress/ skirt: $20 - $40
The skirt lining and the outer skirt are charged separately.
Sewing up a slit: $15 - $20
Adding an inset: $20 - $30
Fitting: $40 - $80
Adding length to a dress: $30 - $50
Price depends on whether you already have fabric to be used or whether we need to find some.