I have a 300w Dick Smith jobby which will run a small tv that size seems to run around the $100 mark depending on quality and the prices head north very rapidly from there.
You will need to know what your maximum power consumption will be and buy a size that will supply that much power. BE aware that you will need to run wiring and fuses capable of handling that much current.
I set mine up to handle a max of 30 amps with 40 amp wiring (6mm I think) to minimise voltage drop.