When I started building my machine, it was with the intention that I could find out what this kind of machine could actually do (nothing beats trying it for yourself, no matter how much you read beforehand) and so that the most expensive bits would be reusable for a Mk2 if that's the way I wanted to go. To give you an idea of the scale of my machine, it has about 1200x600 cutting area, and maybe 150 Z travel. It's essentially the JGRO design (lots of info about this with a bit of googling) but extended slightly. Basic frame is a couple of sheets of MDF - £30? Guides are mild steel tubes and skate bearings - £60? More nuts and bolts than I would have expected, plus bits of angle aluminium and other small components - £50? Leadscrews are stainless studding with home-made acetal anti-backlash nuts - which are one component that actually works quite well! The nuts, that is - the leadscrew material is much too small diameter and severely limits rapid travel. But it was very cheap compared with ballscrews or even trapezoidal thread leadscrews. So, that was the cheap bit, and has to be regarded as throwaway as a better machine just wouldn't use any of these bits. Not even the nifty adjustable tube holders I made on my 3D printer!
The expensive bits are the drive electronics and motors - but these are all reusable for the Mk2. When I bought mine you could still get analogue drivers which were a bit cheaper than the digital drivers now supplied (although the digital drivers are, by all accounts, rather better than the analogue ones that I have). The nearest equivalent to what I have is the 3-axis high-voltage kit from Zapp which is about £500. I built the electronics into an old PC case, reusing its power supply, and I use it with an old PC running LinuxCNC (which is free!). Oh, there's also the spindle - 2.2KW water-cooled, which is probably the most popular option even if hopelessly over-powered for my machine, and mine cost around £350 from CNC4You (UK supplier) although there are plenty cheaper on eBay.

So, round figures - maybe £1100? Of which about 80% is going to be reused in a much better machine (I'll be going for welded steel). For a smaller machine, you could use lower-spec drives and spindle (yes, maybe a router but I wasn't too happy with any of the ones I looked at - and the water-cooled spindles are so much quieter in use, will run all day without any issues, etc). For PC milling, engraving and similar, not heavy cutting, an aluminium extrusion-based machine might make sense, but I've no idea of what this might cost.