Did he say where the leak is ? If in a fitting or line, simple and cheap to just repair or change that out. If in the exchanger coils, commonly they have to be replaced unless it is in a reachable spot that can be brazed. If in the compressor (compressor bypass leak) then that is a major component, and probably the most expensive part of the system to replace, so in that case you would be putting down a lot of money to fix a system that is out of warranty and is probably 65-85% of the way to its average service life of 12-15 years.
One other possibility which he may have been thinking but did not say, is if it is an R22 refrigerant system, that is costing hundreds of $ to fill now that it is going out of production, and will cost more in the future, so he may have figured you were better off with a system that uses the replacement refrigerants. For more on that subject and when the phaseout takes full effect, see the discussions in Home - HVAC (link right below your question, above).
I would call him back and find out where the leak is (if he tested for that, rather than just checking the pressure and saying there was a leak somewhere), and why he recommended a new unit over repair.
Then you can decide what you want to do, and if you wnt to get another opinion. You might want to check his firm's rating on Angie's List too, to see how much weight you want to give to his opinion.