So I've got some prices from Chai for the different bearing sizes:

2 * RM1610 @ 1800mm with 2 ballnuts = $130 Air shipping $76 Total $206 (£128)
2 * RM2510 @ 1800mm with 2 ballnuts = $142 Air shipping $97 Total $239 (£149)

Total price works out like this:

2510+34 Setup
Ballscrews: £149
Steppers (86HS115-4208): £160
Drivers (CW-8060): £126
Total: £435

1610+24 Setup
Ballscrews: £128
Steppers (60BYGH401-03): £78
Drivers (CW-5045): £87
Total: £293

Difference: £142 extra for larger screws.

However, from what I've read and what's been said here I'd be better off driving the steppers in the 1610 example on 70V which would mean using the CW-8060 drivers. That reduces the difference to £103. Note: I've not researched the best drivers, I just picked the ones CNC4YOU had.