Yes it's really a how long is a piece of string with the vets bills - mine are both bog standard, not particularly well put together 14hh cobs under 15 years old and one has never had a sick or sorry day in her life and not cost a penny in vets bills over than standard jabs. The other must have racked up 4-5 times her actual value in bills (thankfully covered by good insurance although we're reaching the end limit now), she's had field injuries needing stitches and subsequent cellulitis, a foot abscess needing antibiotics, 2 bouts of colic caused by breaking out of her paddock and eating god knows what, and most recently lameness in first one and then the other hock, which is looking like it might lead to her being permanently crocked
. Pretty much all random/disconnected things that couldn't have been predicted, other than maybe the hocks which have always looked dodgy (although same could be said for a lot of cobs and not all go lame). I think the PPs who said don't assume cobs and natives are automatically totally sound, bombproof and easy/cheap to keep are right, not all are, IMO particularly those that have been bred and raised indiscriminately/on the cheap like my two, it can ends up being a gamble on whether they are conformationally sound enough for a lifetime's work, plus issues with weight management, laminitis etc... not to put you off OP, I love cobs and they can make super family horses but just do be mindful when you buy, don't buy anything suspiciously cheap, always get a 5 stage vetting and good insurance and budget to always have the excess available (and then some).
Also do budget for regular lessons for you all, and if your DDs want to start competing even at a low-ish level your costs will really start to rack up, running or hiring transport is pricey plus extra lessons, clinics, outfits etc. They will probably start to nag you for more than one horse soon enough too, as otherwise only one can do a class, a PC rally etc. And heaven help you if they start to want to go affiliated or do PC teams, you may as well empty your bank balance now
. Seriously, this is why, whilst there are disadvantages to being on a big riding school/competition yard (more expensive than a little friendly DIY and facilities usually packed out with lessons esp weekends/evenings/school holidays), I'd recommend it. If you can find somewhere than has a lively social scene, people to hack out with, on-site clinics and competitions, PC rallies, people to share lifts/transport with and so on, your DDs will be happier, make friends, can be on yard with less supervision when older and have much less need to box out to all these activities and have more than one horse, since the DD that doesn't ride yours can borrow or hire a horse on the yard so they can both do the same activities...