As Dan says, the prep work is worth more than the coating. Subsequently that's going to be the expensive bit and it will depend on the state of the paint going in as to how much work that will require.
As for a coating, it depends entirely on what coating you're going for. You can go for something like Gyeon CanCoat for £30 with enough to do the car several times over, or you can pay £150+ just for materials for an accredited detailer to do something much better for you. And that's just one brand. Without polishing first, you're looking at a good 3-6 hour's labour + materials.
I've used a few coatings myself but never paid anyone else to apply one for me, but then I'm quite confident to do these things myself. If you're less confident, it's definitely worth getting someone who knows what they're doing to do it for you but if you fancy having a go yourself it's really not that difficult - just understand the instructions, stick to them and work methodically.... and spend hundreds of pounds on new microfibre cloths instead. Oh, and you'll need somewhere indoors to work too!