I thought about doing it years ago! I did a little research, i.e. rang a female private detective (got number from yellow pages I think!) on the pretence that I thought my partner was 'up to no good' with another woman. She told me her rates were ?35 per hour plus 'expenses, which could mean anything!' (this was about 8 years ago btw)
She wasn't going to shag him for that tho', (nor would I have been into the shagging bit if I had decided to give it a go.) The fee was for her to obtain evidence, either by photo's and/or voice recordings etc.
I'm too exciteable for that kinda thing! Would've been 'tailgating' the poor guy from the word go, in case I lost site of him. Not the 'slowly, slowly, catcha monkey way that these guys work!
W.x