It's most likely a late 1974 750 GT. "Retail" on a 750 GT that's been RESTORED could be in the teens. If it's a Sport it could go to mid 20s.
Before you get too excited you should know a few things. "Restore" is not the same as "rebuild". Bikes that are simply rebuilt, i.e. not in stock form with regards to paint and components, are not worth as much.
And now I'm going to make a few assumptions... having said this is your second bike ever, you haven't got any experience rebuilding them. So you're going to need a lot of help, as in, you'll have to farm out a lot of the work.
This gets expensive fast. If you are truly wanting to restore it, you'll need to have the work done by people who know what they're doing. There aren't many, and they aren't cheap. Even rebuilding it you have to find a shop familiar with them, and not every town has one.
Maybe I'm wrong, you're quite the talented mechanic. Great. Is the engine complete? If it is, and you are doing the majority of the work, budget about $5000 for the engine rebuild. If the engine is missing a sidecover, add about $1200 to that number. I don't know how experienced you are with engines, but the prices on bevel parts will about choke a guy who's used to working on small block V8s.
Bottom line, if you want to do a decent rebuild on a roundcase (A 1974 or earlier bevel twin) you should be willing to spend $10k doing it. More if it's a basket case. Less if you're ok with using spray cans to paint it, reusing parts that are "probably" good, etc.
Still with me? Great. So here's the good news. The bevel Ducs have a very loyal following and there is a huge amount of support out there for people who undertake projects like this. For starters, try the bevelheads mailing list.
http://micapeak.com/mailman/listinfo/BevelHeads/
There is a huge amount of experience on the list and the people there can help you avoid costly mistakes.
There are a few books you can look at to get an idea of what you're dealing with. A couple you might start with are;
The Ducati 750 Bible (Ian Falloon, who's on the bevelhead list)
The Standard catalog of Ducati Motorcycles (Falloon, again)
Before you even touch the bike you need to buy a manual. Maybe two manuals. DO NOT start pulling the engine apart before you research what's inside. It is NOT a small block Chevy, you can't just rip it apart, install a $1000 rebuild kit from JC Whitney, and bolt it back together.
There is also a parts manual you will need if you decide to embark on the journey. They are worth the cost, trust me.
If you are serious about rebuilding this, buy the 750 Bible and start reading. Get on the list. Start saving your money...
Contrary to what it might seem, I'm not trying to talk you out of doing this. I'm just saying it's going to be expensive, and it's going to take a long time.
I started with a more or less complete but non-running bike. I've been playing with cars/bikes for years, have access to tools most people don't, and it still took me over three years (I took time off) and $10,000 to rebuild my bike. I figure it's worth about what I have into it, but when I started on it I was fully expecting to have about $4k more into it than what it was worth (prices went up, a lot, while I was building it)
Also, there are people who will tell you they've rebuilt GTs for much less (Hi Tom!). Yes, and those people typically have experience and/or aren't rebuilding the engine.
It sounds like you have the start of what could be a great project there. I just wonder if you realize how much of a project it is... If you take it on, more power to you. If you think it'll be like putting a 350 Honda together... um... we need to talk...