Let's see, ignoring palmtops and computers that have never been the main one (a Lynx, a CP/M box with 8" floppies, a Mac, a QL, an Archimedes, a…)

I've gone from two 4MHz (ish) Z-80s, one with 16k RAM for £120, and one with 48k RAM also for £120 a couple of years later..

.. via a 2MHz (ish) 6502 (effectively the same speed as the Z-80s) with 48k RAM probably for £399 (and free 5.25" floppy drive when Lasky's lost the credit card details) ..

.. via a 4MHz Z-80 with 128k RAM that I'd like to have working again for about £400 with one 3" and one 5.25" floppy disk drive..

.. to a 8MHz 8086 with 640k, for about £250..

.. to a 20MHz 386DX with 4M and a 40Mish hard drive, for about £1k..

.. to a 33MHz 486DX2, 4M to 16M – that cost £250 on its own – to 48M and a 1G drive and also upgraded to a '66MHz' 486DX2 and a '99MHz' 486DX3 before settling into the '120MHz' Cyrix M-2 that's still in the flat, for about erm, probably almost £2k if I could add it all up..

.. to a 450MHz AMD K6-III that's now here at work with 384M and about 40G for about £600..

.. to a 2GHz P4 with 1.25G RAM and 120G+40G drives for about £500.

Note that the last one was the cheapest since the 8086 and after the beginning, I've only tended to get the next one when it's been a) around for about a year and b) about three times the speed of the current one.

It's been similar with printers – although that's because since HP Laserjet Series II and III printers became available for about £20 second-hand, I've been happy with them.