Can i run power to my workshop using an open existing 30amp breaker?
You can put in a sub you need 4 wires ground , neutral and your 2 Hots. Since the shop is detached you will need to have a ground rod and #6 wire is normal for you his , I would consider running a larger feeder wire than #10 but if only running 1 saw and your lighting #10 could work. Even though you have the wiring protected at the main panel with a 30 amp breaker your sub can have a total of more than 30 amps if you add the breakers. For example if all single pole breakers you may have 80 amps if you total all the breaker values. The total load is the important thing here as we usually have a safety factor and only use 80% of the circuit capacity. In fact most homes that I check are running at 25% of their main breaker or less.  So yes you can but I would go bigger, I have never been asked to reduce the size of a panel but many times I have gone back and upsized the service. At a minimum put in larger conduit so you can upsize later. Added
I was realizing that you did say 2 with ground as I earlier mentioned you will need 4 wire to the sub since you will need to add a neutral if in conduit back to the panel this or a new 4 wire run at the largest size you can afford, for example I just put a sub in for a small heat pump it needed a 20 amp circuit , I put in #6 and a small sub, now I have plenty of power to add lighting to my round pen and even a hot tub over there or whatever I might need hope the additional info helps.