I was originally asked to work 40 minutes strolling at a dinner and dance at tables but the organizer has decided people might get annoyed when not at their table. So he has asked to change to a half hour cabaret act for the same price. The problem I have is my other fellow magi have said they charge more because of extra hassle and setting up. Any advice will be great. Please feel free to PM me if you have advice on sort of prices to charge.

Set your price and either do it or don't. What difference does it make if you have to set up?

Just in case I need helps with props on and off stage. also help with mic and sound, music. the problem is do I let him down even though he changed what I was doing.

Be up front NOW - you need to call him, be professional, be polite, explain what you normally charge for for a stand-up show and WHY it's more expensive. This is only applicable if you actually have a stand-up show with music, dancing girls, stagehands, etc. If you don't deliver the extra cost then the client is going to be even more upset . . .

I usually charge the same price anyway, can't take another booking that night. Also if I'm doing close up I'm usually working longer than if doing a cabaret spot. If there is a big difference between the prices for either, you should consider narrowing the gap.

Paul.

First, do you do a cabaret show? Are you experienced in that genre? If not, by all means tell them and don't change the gig. Many a magician has assumed that because he can do close up he can therefore do cabaret or stage (and vice versa) when he has no experience in that ares, and as a result has failed miserably.

Assuming you are competent at both genres, were I in your shoes, I would simply explain that I have a different fee structure for cabaret shows and that an adjustment in price will be necessary. I would tell them what the fees for each type of show would be if purchased separately and then give them a deal for the combination show.

For example, if it is an audience of say, 150, my normal fee for a 30-minute cabaret show is about $350. My fee for an hour of strolling is $250. So I would explain that I need another $100 if they want to change to just the cabaret show, but that I'll give them a 50% discount if they'd like to add an hour of strolling during the cocktail hour, etc.

That is what I would do, and it works for me--I can usually upsell them and get both, so I end up making about $500 instead of only $250, but they still feel like they're getting a really good deal! You may choose to go a different route. That's totally up to you. Do what makes you comfortable and what will make the client happy.

40 minutes of walk-around doesn't seem like a lot of time, depending on the size of the venue. My prediction is that you wouldn't be leaving after just 40 minutes, because you are a nice person and would feel bad about leaving without hitting every table...
(Customeers with experience in hiring magical entertainment know this, by the way, and sometimes take advantage.)
A cabaret show is finite. When it's over, it's over. Buh-bye.
Personally, I'd make the client happy this time, then tighten up my letter of agreement/contract to avoid repeating the situation.