Ok, did a quick conversion and £90 seems to be around $120 (today). There are loads of PC speakers available on the market and bear in mind that you can pair almost any Bluetooth speaker with your computer nowadays - provided your setup is not too old and is Bluetooth enabled. The original Sonos had no Bluetooth capabilities - which was annoying, to say the least! - but almost all the more recent models do. So, you could keep building on your Sonos configuration and keep everything nice and similar.
I guess the issue might be that they're not going to come in under your budget. So, if you're looking for a purpose-built pc speaker, take a look at
this list on Outeraudio. At under $100 I like the BOSE Companion for it. I also like the PreSonus Eris - also under $100.
Both of the speakers above are "wired" speakers, which is a bit 1980. Looks like most of the wireless speakers on that list are going to put you out of budget, but you can find some decent Logitech's there. Finally, remember you can filter by price on Amazon... ;-)