It really depends on if the WG is into you or not. I've been on dates with WGs before and it can be tricky when on these dates I usually pay for everything, eg. food, movies, accommodations, sometimes shopping for reasonable items. Some actually fight to pay but I usually insist.
Usually I don't offer money to the girls and sex is not always a given but if it happens bonus. I have tried to give money before after sex the response can be quite different ranging from girls just taking the money to girls getting upset and very mad and saying they are people too and the only reason they went out with me was they liked me and wanted to have a good time and not be treated as a WG. So I guess you'll have to use your instincts.
I think if you invite them to have dinner or a night out they probably don't expect you to pay them for their time but some can be sneaky and give you a sad story or try to get you to buy them expensive stuff like cell phones and designer bags, and if sex does happen I sometimes offer them money and say it's for a gift and not for the sex.
How much money did I give the girls ? Well it depends if I had a good time or not, did I buy them gifts already, how much time did they spend with me. A whole night out from 7:00 pm till the next morning I have given anywhere from 2000 HKD to 6000 HKD again some took it and some didn't.
Hope this helps. Good luck