I think it is a bit early to tell.
But to get an idea of a possible pricetag - take a look at the Spectrum Next sold here:
- £210 for the machine ex VAT for the Spectrum Next.
(Not at cost, sold with a 20% margin on top)
My guess is the price for the Mega65 will be the price of the spec next - and then some.
(due to a larger FPGA and more materials used, because the mega65 will be physically larger in size too.