That’s buying cheap fizz at around £25 a bottle
I think we must have different definitions as to what is 'cheap fizz'.
Given that Champagne attracts a significant price premium but many people can't really tell the difference between that and other fizz or actually prefer another type of 'fizzy wine' and there are many other styles that are considered to be as good, if not better than 'cheap' champagne, I would really question buying champagne when catering for a crowd.
For example, I bought a selection of Cremants and Cavas from Sainsbury's that were reduced from £10-12 per bottle to £8-10 per bottle and were in the 25% off when you buy 6 deal, so came in at an average of £6-7 per bottle and that's still not what I'd call 'cheap', more good value for what I got and a bit of a treat.