Personally, I'm more concerned about avoiding getting models I
don't want in a group, than how much a batch is going to cost. Like Saarlooswolfhound said, there's not really an upper limit on how big a batch is too big, or too expensive - if I want all the animals in it I'll happily add up in my head roughly how much I'd pay each, and if that's less than the current price, put in a bid or buy-it-now. I'm even fine with duplicates of the same model, cos I do plenty of repainting and am pleased to get spares to customise.
But I
wouldn't buy if it was a mixed lot of different species all in together : I really only collect horses, so I wouldn't touch mixed lots, or any split simply into whether they're farm animals or wildlife.
I'd also be averse to lots containing anything really hideous or cheapo - animated movie character figures, the girly child's-toy horses with pastel colour manes and makeup on, or tacky plastic macdonalds/poundshop-looking horses, I wouldn't buy a lot with those in even if it contained a few sensible 'grown up' style models as well.
I'd mind less if it was a batch of similar-style models from different brands, like CollectA and Mojo horses, or Papo and Schleich donkeys, cos the chances are I'd like both, and if it was a group with one odd-one-out model from a brand I didn't generally want to buy, I'd just tolerate having the unintended extra if it was an ok mould, and if it was one I really didn't like, give it away with a 'free to a good home' label round it's neck on a sales table at a model horse show at some point in the future when they're running again.
So the listings I wouldn't even look at would be where someone's put a whole multi-brand batch together cos they roughly match in species/breed, or jumbled up lots of species, and there's loads I'm not remotely interested in acquiring with one or two models I'd have liked.
Even if they're so cheap that all the unwanted models are more or less 'free', I don't want that amount of stuff to store and eventually try to give away. I can't get a seller account on ebay cos I don't have ID, and can't use any of the phone-app selling sites cos I don't have a phone, so my only way to get rid of stuff would be hoping any of my friends want it (which is unlikely cos we all collect horses, like more or less the same brands, and avoid the same ones for the same reasons!
), or I could try selling here on the forum but I suspect I'd still end up stuck with a backlog of ones I was trying to shift, plus hour-long walks to the post office are annoying, so it's easier to just avoid buying mixed-brand lots.
So my idea would be splitting lots by animal type
and maker, but species first and foremost, and brand slightly less important if they're
similar. I know a lot of people are less fussy or have no issues selling on, and would happily buy mixed-brand bulk lots, so this is just one awkward collector's take on what makes a good lot and what doesn't