We have had lots of experience over the years of holidaying on a shoestring, preferably for nothing. When we were first married we used to volunteer at the community where we had met and counted on those few weeks away to pay off the overdraft in savings on gas and electricity. We did have to do some work but in exchange stayed in a lovely place with lovely people and the only cost was the train fare to get there. You can do something similar now by joining the WWOOF network, willing workers on organic farms.
The other alternatives which cost almost nothing are staying with friends and family and having a stay cation. Over the years we have found that staying with friends and family works well for a couple of nights but any longer and you are in danger of out-staying your welcome. Staycations are good if you really treat them like a holiday and do not use the time to do jobs around the house. Exploring your area and visiting all the attractions nearby can be great fun and you get to sleep in your own bed at night. If you want to make it really exciting you can even camp out in the garden or the sitting room for one night !
Something similar to a stay cation is to do a house swap and we have done several of these over the years with mixed results. They have always been in beautiful parts of the world but sometimes we did come home with fleas and on one memorable occasion our house got infested because the swappers had brought along their dog !
Camping and youth hostels were very popular with us when the children were smaller but the prices have crept up and now I'm not so sure if that is all that cheap if you have a reasonable sized family, for instance a family room in a youth hostel can now cost over £70 which is more expensive than a Premier Inn. You can go free wild camping in Scotland and on Dartmoor but with a family the lack of facilities for more than a few days can be a bit restricting.
Our new favourite way to travel is to use AirBnB, we have always been lucky so far and can often get a flat for around £30 a night which for 3 of us is a very good price and cheaper than many campsites. A full kitchen makes self-catering easy and you do not have to set up the camp when you arrive !
If you can travel out of season you can also find a bargain, for instance I saw 7 nights with Eurocamp in France in the first week of September for £210. There are holidays for frugal budgets out there, so start saving for next year !