There are several threads of USB vs Ethernet motion controllers - it would be worth researching these. I've never used a USB controller (though the seller of the Starmill that I picked up did). I've the ethernet version of the first link (a NVEM controller), which is okay, works, but nothing special.

Ignoring the prejudice against the USB motion controllers (and many people do use them) then they could be an affordable alternative to the parallel BoB.

Regarding your budget - remember, the BoB or motion controller is only a small part of the overall solution. You might find that you need to buy stepper drivers as well - at a cost of £20 upwards each. This, and the underlying PSU will help to define the performance that you can expect from the machine - you need to go into this with eyes wide-open on the likely costs.