A bouncy castle costs about £50 so not a particularly cheap option but they are great and keep the kids entertained. My son's b'day is mid march and we are putting a bouncy castle in the village hall, cost is about £100 for hire of both which is cheaper than soft play.
Will the kids all be the same age?
I think if at home and you don't hire a bouncy castle - the current weather might put paid to that anyway, then I think you just need to have a plan of games. Pin tail on donkey, piñata, musical bumps/statues, pass parcel etc.
If they get fractious you could always put a DVD on like Toy Story and have popcorn.
We had a lego themed party at home for DS's 5th birthday last year. Had 9 boys aged 5-7. Just did party games - pass the parcel with silly forfeits, pin moustache on lego man, musical bumps, musical mats - then split them into teams for lots of relay races in the garden (hopping, sack race, space hoppers, dribbling a football etc) which went down well. They seemed to really enjoy it as don't seem to go to many parties with traditional games (lots of soft play parties). We had a bouncy castle the previous year which also went down v well, but this was much cheaper!