Hiring a venue can be expensive but there are easy ways to keep costs down here. You could host the party in your home, if it is big enough and not hire anywhere at all. Alternatively you will find that village halls, church halls and scout huts can be pretty cheap to hire and are likely to have a kitchen as well as tables and chairs, which means that costs could be low. You will find that some even have an outside space that you could use. If you have friends or family who have a big home, then consider asking them if you can borrow it for your party.
The cost of food can add up a lot for a party. However, you could ask guests to contribute if you feel it is appropriate. Otherwise you will need to make sure that you choose food which is cheap and also shop at the cheapest place to get it. There are many discount supermarkets you could consider using or you may even be able to use a wholesaler if you have access to one. Look out for sales leading up to the event and stock up on cheap things but check use by dates so that you do not waste things. Plan your menu carefully, making sure that the food is good value. Caterers can be really expensive so it is much better to do the food yourself, perhaps buying things like premade sandwiches, salads, big cakes etc from supermarkets, cutting them up and putting them on serving plates.
If you think that drinks will be expensive you could hire a venue with a bar and guests can buy their own if they want to. Or you could ask guests to bring their own drinks. If you are having alcohol, you may need to get a temporary licence from the local council that is if you are hiring a venue, so make sure that you check. It could be wise to limit the amount of drinks you buy because of the cost, but also if guests have less to drink their behaviour will be better and there is less likely to be damage or mess for you to clean up. Have a good selection of soft drinks, which are cheaper, so that guests are not always tempted to go for the more expensive alcohol.
The cost of decorations can really add up. You could buy things that you can use again so that you will save money in the long run. Alternatively go cheap but know that you will not be likely to be able to reuse them. You could always do without decorations altogether so that you can save money on this part.