My engine was running very hot and my coolant was nearly empty and steaming about 7 days back. It had been fine for months before then. I went and baught some new coolant, mixed it with demineralised water, and filled up the coolant container. I check it a few days later and its empty, I refill it, check it a few days later, empty, refill it, check it again, empty. After 3 times I'm thinking that there is a leak. I then check the radiator cap (above the fans I think?) and I can see the green coolant, I stick my finger in and its nearly an 1 Inch deep. Would you think its leaking from the container or the radiator? My plan is to see if the radiator empties out eventually. There is a wet patch underneath where my car is so its definately leaking. I'm also getting hard starts if that means anything. Thankyou very much for your time.

Engine: 5S-SE - 4 cyl manual
What should I do? Will it cost much to put it in to get fixed?

I'd bring the car to a shop and have them pressure test the system, the leak will pop out really quick.
don't drive the car anywhere but to a shop. You probably have a busted hose.