We don't know what restrictions were in Rupp's contract with John Hancock. So even the appearance fee is a guess. Each athlete can negotiate with JH and the beauty of it all is - only the agents know who is getting what and there are probably non-disclosure clauses in the contracts.
Standard inserts could be completing at least 30K, running under a certain time or finishing in a certain place to receive certain sums. Additionally, not running any other marathons for a period of time could result in a penalty as well.
I would expect Rupp's contract to have a little more leeway than Kellyn Taylor's in terms of both the total appearance fee, and the penalties for non-compliance, because his agent had more leverage. Rupp likely was courted by London because he is the most recent Olympic bronze medal winner, the Chicago champion, and the top American marathoner. To the casual observer, he'll draw more eyeballs than a non-descript African runner who merely has a stellar PR time behind him and no Olympic/WC medal or top 3 finish in a major (for instance, a runner who finished Dubai in 2:04 but came in 7th ....). Rupp's agent, Ricky Simms, would be able to play John Hancock and Virgin off one another to get the best deal for the client. Even if Rupp wanted to run Boston because a US win would win more for his brand and for Nike, if London came along with $500K or more in appearance fees, that would be pretty hard to decline.
With respect to his running Prague - a previous poster mentioned in the Kelly Taylor thread that the penalty issue may explain why Rupp was announced, withdrawn, and then re-announced. It's possible Prague made up for the appearance fee .... perhaps Nike paid it .... or perhaps John Hancock negotiated some kind of 2019 appearance in exchange for its agreeing to waive the penalty. We don't know. Only people who reliably know are Galen, Alberto, JH, and Ricky Simms.
Kellyn Taylor mentioned the 60 Day No Marathon - restriction. Presumably that was only for her appearance fee, but who knows what was stated specifically in the contract. Regardless, she had less bargaining power. She has one US road title (25K in 2014), 6th in the US marathon Trials in 2016, came in 4th in the 10,000 US Olympic trials that same year, and in 2017 ran her then PR marathon in London, 2:28:51.
She is a very good runner, but realistically, JH and Virgin weren't going to go nuts trying to get her signed up for their races in 2018. Basically, she could be presented with a take it or leave it contract ....... To her credit. She seems a lot more likable than most pro runners - including Rupp.
This fall, she'll be more in demand with a 2:24 - and hopefully she can capitalize on that. But in 2019, without more, it's not like JH and Virgin will be climbing over each other to sign her up. Not when Molly, Desi, ....maybe Shalane ... and Jordan Hasay are out there.