You thinking frameset or full build OP?
For frames maybe a plug 5 But by the time you buy a fork it’s probably about on par price wise with the kaffenback frameset…
Any real cost savings will come from how you build the thing up and what sort of weight/function/price trade offs you can live with…
Assemble a 1×7/8/9/10 drivetrain or buy a full group?
Use an old pair of cranks from the spares bin perhaps?
Cable or hydraulic brakes?
Barend or “brifters” for shifting?
Cheapest finishing kit you can find or a bit of bling?
Basic £100 wheelset or spend a bit more?
You might have some parts/sniff out some bargains but it’s hard to do a bargain build and really keep the spend down IME.
PX only do a Rival 22 build with BB7s for the kaffenback right now and they want £900 for that. You can probably do better…
Compare it with an Arkose 3 (£1200, 5800 hydro group), Arkose 2 (£1k, 4700 hydro), or Arkose X (£900, hylex and 1×10 Deore) all with aluminium frames and Carbon forks, on balance those are better VFM in terms of spec and weight Vs price and service backup…
All IMO of course…