Bad cooland leak

I have a pretty severe coolant leak, all coolant leaks out after 1 short journey, however there is no sign of any fluid leaking out anywhere (nothings wet). I stopped driving it and the system was pressure tested. This showed no leaks so I was advised it was a gasket/head problem. Now I've taken it to my garage they think it's the radiator and have quoted me a lot of money to fix it. Does this sound right to you? Surely if it's the rad there would be some sign of seepage after a pressure test? Also considering the amount of fluid that's disapearing I'm guessing a sealer product would not work. The vehicle is old so I'm not too keen on spending lots. Thanks