We've done 3 whole class parties for between 30-40 children each time and I would say £500-£800 is about on the mark.
First one - hired our local hall for 4 hours (£180), bouncy castle and soft play hire (£200), cake (around £80), 40 party bags (£120), food (we catered it ourselves - sandwiches, crisps, biscuits, fruit, veg sticks) (£120), decorations/balloons/games (around £30).
Second one - hired soft play, exclusive use for 30 kids. Cost around £500 (food included), plus cake (£80) and party bags (£90).
Third one - trampolining at local activity centre, around £25 per child (food included), cake (£80), party bags (£90).
The cheapest whole class parties we've been to have been park parties in summer time. They've been absolutely lovely and lots of fun but unfortunately my kids have winter birthdays and I'm not sure I can convince anyone to come stand in our waterlogged local park for a couple of hours in November!