I don't have one, but have looked into them.
The cheapies (£20?) need a team to put up and take down. "Pop-ups" are a step up in price. Aldi had a pop-up for about £60, which was cheap for what it was.
Best advice is probably to watch out for end-of-season sales. Talking of which B&Q have a 'clearance event' starting today (with different stock likely to be found in different branches, so, if you are passing …)
And as a really weird one, next Monday Lidl will be offering a set of three folding tables (each 1m x 60cm) "use individually or combined" for £39. (Looks like they clip together after folding for storage/transport.)