Build cost of a Gryphon – not sure but a few hundred more than a regular 26″ frame build as the particular components are in less supply and tend to be a touch more.
Careful secondhand and classifieds buys would make a big saving. But, again, the stuff you’d need comes up a bit less often.
Superstar do some of the cheapest (and so far fine) 29er wheelsets that I found.
STI’s are rather expensive and their cable pull means you either have to buy road cable disc brakes (IIRC Tektro or Avids). Alternatively any old road brake levels and go SS (not for me) or bar end shifters (cheaper than STIs).
Normal MTB cranks, rings, cassettes and BB’s all work
At least you’re saving money on the build (compared to a 26″) by not needing suspension forks.
Factor in funny bars as well like Luxy’s or Woodchippers.
There’s secondhand 29er tyres to be had as well. Plus tubes, bar tape etc.
Don’t let it put you off. So far mine’s been great fun. It is handy to give it a long build time to source the best and cheapest bits though. Otherwise buying new, off the peg stuff would probably be (guessing wildly) £1,200.