C.M.
The cheapest and best party we did was at a park. We rented the pavilion for only $50 and there was playground equipment and a splash pad nearby. We didn't even need to entertain! We had brought games to play but the kids just played on the equipment themselves. We had grilled hotdogs and chips for lunch and birthday cupcakes. Cleanup was just throwing everything in the trash.
We did house parties, and those end up expensive if you consider the cost of the food, then you have to clean before AND after the party. Kids are messy! We rented a bounce house, which was a big hit and perfect, but it still was $150 for the rental, plus the food. So $250 and you're still stuck with the cleanup!
We recently did a skating party. It was GREAT in that we didn't have to do a thing, they did everything. It did cost us $200 though.
The party at the park was the cheapest and easiest. Yes, you are dependent on the weather though, that's the only thing. I will say we didn't need to entertain and cleanup was a breeze!