I wouldn't be insulted. I don't know much about EBA's or cichlids in general, but from a quick google it sounds like EBA's are not rare, not that difficult to pair up, and not difficult to breed either - which is great in terms of making breeding pretty easy, but also means that a bonded breeding pair isn't necessarily worth hundreds of pounds either, since it wouldn't be hard for him to leave a bunch of acara in a tank and wait for a bonded pair to happen himself, right?
I couldn't find much info on raising EBA fry except that the parents are usually good at doing most of the care, and that they need a separate tank. But, you can't value a fish (or pair) based on what they could produce over the next ten years. They might only live for eight, or one might die after two years or something, and with most fish, not all of the fry make it either. You're counting each fry as pure profit by the looks of things, but not counting the cost of having them in a different tank, feeding them five times a day, the increased water changes it takes to raise healthy fry, electricity to filter and heat and maintain another four foot tank, time it takes to list and sell the fry and interact with each customer... Although when you say you were happy to think of the value as 10% of the cost of each fish they
might produce, maybe that's how you're accounting for costs?
Why not separate them into another tank to breed, if they're very profitable, and they're beating up your other fish? Will it cause the other fish and the acaras a lot of stress for a breeding pair to be in a community tank? I know cichlids are cichlids, but I don't know whether that's something you should be avoiding by moving them,
@Wills ? Will it be too stressful for the EBAs or the other fish to remain in the same tank?
Anyway, he could be as crooked as the day is long and trying to snow you, but it sounds like a pretty reasonable offer to me, if he usually sells them at 10-15 each, and they're not super hard to pair up. That makes £75 store credit seem a good starting point. He heard that yours had paired and were breeding but still in a community tank, you're not wanting to set them up as breeders yourself, so he offered you what he'd be willing to pay for them, but you could either suggest what you think they're worth, or just say no

I just think it sounds maybe a bit lowball, understandably, but not outrageous either considering the costs and hassle of fry raising and private sales. If it was pure profit with no downsides, you'd be doing it yourself too