One of my clients has asked for a threesome with a bi guy. I said fine. I don't know anyone up for it so he suggested a male escort which I was fine with. He wants to watch the male escort have sex with me first.

Now that I've thought about it a bit more the escort is getting it for free with me plus getting paid by my client.

Has anyone done this before and how do you work out what should be charged. Should I charge the escort.

If the male escort is a professional like you, I assume he'd see having sex with you as part of his job, for which he's getting paid by your client, i.e. it would be a bit bizarre to charge him. You just need to make sure you're happy with your fee.

If the client wants to involve another guy then he should be paying you the extra for your interaction with him, if the other guy really is an escort and he has to pay him a fee too that's his problem not yours. Why should he get his fantasy at your expense?
 Special scenarios cost and if that's what he wants to see then he's going to have to pay for it.

  hope it goes well Jessie!

If you're totally happy with the fee, cool.
If the deal is he's a voyeur who doesn't participate in activity and penetration with you...and he sticks to that agreement without wriggling out of it in the session, cool also.

It's your choices and what you feel comfortable with.

I guess I need to remind him on the day as sometimes our best plans go out of the window in the heat of the moment

Yes, I suspect you'll need to do that more than once.

A phrase that springs to mind is, 'Champagne appetite on a beer budget'. When a fantasy is modified like this due to financial restraints the punter can set himself up for disappointment. Usually they offload the blame and resentment onto the provider(s).

You know him already, so if you predict him doing a bit of emotional blackmail for unpaid services at the gig...you're probably right.

As your client has suggested a male escort of his choice; I would be charging him for an MMF (i.e. the price you would charge for a 2 guy booking). It's no different than if he invited his mate along.

The client will therefore be paying the 2-guy rate to you, and the male escort will be paying nothing (and possibly be receiving a payment of his own through the client). That's fine, you're still getting paid for two blokes. Don't absorb the costs, or charge any rate, to the male escort. Payment is the client's job; the male escort is a third party like you are.

Charge the client fully for 2 guys, at a rate that will make you comfortable and happy within the booking. It's what he expects and has asked for.