$50 RTK (+$65 shipping)

Plus ~$65 shipping NS-HP-GN5 : PX1125R L1/L5 RTK Breakout Board - NavSpark Store

Some discussion here

I realise you may not want to use it for ‘proper’ Acorn, but software compatibility would be nice for those experimenting on a low budget.

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Looks like there is some Arduino code for it