When needed, I have access to a local friendly travel agent who may not be the best expert on miles optimisation but who in combination with my own skills could still be helpful. They charge - if I remember right - EUR 15 per booking.
I have also used the help of another FT'er who has access to TA software. His agency charged me USD 20 per person per booking last year.
These are roughly the amounts I would be comfortable to pay. In some cases maybe a little more (up to EUR 25-30).