What would you normally charge for these services?  I would add up what he would normally pay me, take that as a starting figure then think about what level of "discount" I might be prepared to offer.  After all, it's a fair bit of money which should come in regularly each month but it also depends how much you like/dislike having sex with him.  He seems to have been clear on how often he'd like to see you except for the "occasional" overnights.  I'd just want to be a bit clearer on how many of these he's expecting.

If it's from someone you've never seen, it's highly likely to be TW.

If you have seen him, and are happy to do this, then consider getting the money upfront (say every month) before there are any discounts.

The best thing to do is set up an amount of time each week he will be visiting plus how many overnights a month he wants and work out how much you would normally charge and give him that price but if you want to give a discount state to get that discount he needs to pay for the whole month up front.
don't let him try the old well I will pay some now and some at the end of the month or I will pay it all at the end of the month.  treat it like every booking, Pay before Play.  That way you wont be giving out cheap bookings to someone who suddenly decides he doesn't want to pay the rest of the months money
also get the agreement in writing just so you both have proof of what is being offered for the amount of time.  Also write it in a diary when he arrives and leaves in front of him so that he cannot suddenly demand extra time saying he is owed it. If you know him well enough get him to sign it

Thank you all! What's happened is he advertised for this. A girl to see him once a week and receive a monthly allowance. We meet for first time tomorrow. I've suggested a basic monthly figure and one off overnight charges which he has agreed sound close to his idea.  We'll have to see how it goes!
By the way, what does TW mean?! Thanks

TW = timewaster

ih of course, thanks!

No I think he is very genuine.

Why do you think that he is genuine?

The simple test is that if he has not paid you for the first meeting within the first ten minutes (in cash, and certainly before he sees you in your underwear or any sexual stuff) then he's after free sex.

Thank you! We have agreed a pay by visit deal now - much simpler and I think it will all add up nicely!
Xx