The 2 precision arrays are way cheaper than most of the stuff of this class, about $3 each for the really bog standard grade, $7 each for the almost perfect, and about $12 for the best you could conceivably need, as you only need to pay for stability not accuracy, Not to mention I have generic 0805 footprints on the back side of the board if you really want to keep it dirt cheap
Even the op amp footprints are standard, you could swap them out for some cheapies if you don't have as good supplier options. I've been trying to add test points and alternative footprints where possible to leave these options.
Heating everything up hurts quite a bit, all the resistances have thermal higher noise, the op amps generally have worse specs in general and increased noise of there own, its generally better to just pack it into a good insulating box with some desiccant to keep the humidity stable as well, (If you look at the datasheet almost everything is trimmed in to perform best at 25C)
Equally the op amps are way cheaper than I had imagined, I'll have to check my BOM, but I suspect the whole board so far falls under $100 including PCB.
The other issue, and what I am trying to work around by layout is making sure every part of the board is the same temperature is literally impossible, but you can minimize how much variance is seen by components and connections. and try to ensure that differential signals (say both op amp inputs) see the same temperature / same line on a thermal gradient map as possible. every connection between dissimilar metals is technically both a voltage and current source depending on what hurts you more.
E.g. the parts of the circuit I went through and worked out the power dissipation for, normal engineering brain would say a change of 25mW is literally nothing, but it causes a peak on the thermal gradient map (something I really wish I knew how to model) and have a slope leading away from that part,
25mW in a SOT-23 package is an 6C rise because the package is 250C/W to pcb, which while not a crazy steep gradient does effect what surrounds it, or on the more exterme, U4, he is dumping out 300mW just sitting on the PCB, but he has a very low noise figure, so its more a case of working out how that heat will effect the things around him, the reference is easy to hide on its own little thermal island, the op amp, not so much.