I don't think a referral fee is an immediate turn off if the party planner brings you a large amount of work. This is fairly common in several different service industries. But I would not let them dictate the terms, I would make sure the contract was beneficial to both parties. I would not be shy amount asking them to change the terms to ones that you are comfortable with.

Apart from the 'fee' they are asking this does seem suspicious on several other different levels. Like the fact that they want a contract before they have ever met you and tried your products. Also, they have already stated that they have several other vendors with this arrangement, so if they have other cake vendors then they are not really going to be bringing you much work, if any at all. (they may have other vendors, but are trying to get a bigger percentage from you?) Or they may not have any contracts and are saying they do to try and make it sound more legit. 

If you do meet with them make sure you ask for references both from vendors they have worked with and brides. I would also suggest bringing someone with you to meet with this vendor, so you can get a better read on them. (if they are caught off guard by you bringing someone else you can see how they deal with unexpected change, which for a wedding planner is a must! they need to be calm and cool under pressure.)

If you like them, and their references check out, and the contract seems okay the last thing you NEED to do is have a lawyer look over the contract and make sure it protects both you and them. Also, there should be nothing in the contract stating you will work exclusively for them as there is no guarantee they will bring you business. Good luck!

ASo for the people who have these arrangements, what is a standard fee? Percentage? Is that before or after expenses? 10% off the top is a big difference from 10% of the profit.

AThere really is no "standard" fee, it depends on the cost structure of the partner and market value for the services they provide, just like when pricing cakes. One example of a mutually beneficial situation would be a baker selling a cake as part of a planner's party package for the same price as a direct sale (or slightly less considering the direct customer acquisition cost is $0), with a markup applied by the planner to the entire package.

A partner requesting upfront payment or agreeing to a deal without sampling your product are red flags.

I don't know about standard percentages, but I do know it is usually of the gross for referral fee to avoid any confusion or discrepancy as to the profit. You don't want people arguing over what job costs are and whether something should be put into a cost for a certain cake or spread over several cakes (say you need a new pan for a cake that you purchase and take out of the profit for that cake to which you pay a referral fee, but then you have a new pan you can use for other cakes and it wouldn't come out of the profit for those cakes which you may or may not be paying referral fees on.)

Take for example a franchise (like McDonald's) in order to use the resources, name, training tools, etc. each franchise pays McD's a % of their gross, not net. 

I am using super vague examples and guidelines as I do not have any contracts for referrals, but know of people that do for different industries. 

Also, if you do have a contract for referral fees payment for the referral should ALWAYS be after you receive payment in full from the client, never up front.

AThis is what I have come across although I'm no expert! 10% or 15% for a high value spend. This is on the total price of the cake, not the profit.

This just sounds suspicious to me.  Like you make this huge cupcake order, get paid, pay this referral fee, then the payment for the cupcakes disappears.  I would not do this, sounds like a scam.  But I may just be distrustful, especially of craigslist.