I have an '85 JCB 1550. I'd say you paid all the money and then some for it. But that really depends on condition. Im my opinion, if you manage to find a really nice one with minimal wear and tear and/or completely rebuilt, then it could be worth that much. Otherwise one in good condition should be available from a private seller for about $6500-$7500 give or take a thou. That same machine a dealer will probably ask about what you paid for yours (after slapping on a 100mph yellow paint job, of course).
One thing to remember about JCB's in the US though: these are very well respected machines abroad and they generally are very well built. But there are peculiarities being a foreign (British) machine that can make them more expensive to maintain and at times make parts availability a problem.
For instance, the hydraulic hoses on your machine have British ends on them. If you blow a hose or want to replace your dry rotted ones with new ones, you usually can't just go down to the local hydraulic shop and have them made up (like you can for a Case, Deere etc). Nope, you gotta get them at the JCB dealer or find and order the specialized fittings from somewhere else and then have someone join them to hoses for you. Gonna be more expensive than common hydraulic hoses on American machines.
On the other hand, the Perkins motor is easy starting, plenty of power, runs fantastic and is very reliable. The shuttle shift transmission is the same (or nearly the same) as that used in Case machines of that time. Overall it's a good machine. The extradig hoe extender adds value if you have that.
A few more thoughts: parts prices can vary according to foreign exchange rates between the British pound and the US dollar. Our dollar is steadily losing value due to our $17 Trillion debt (and climbing), but then the pound is being debased as well (along with the euro). But if the dollar really starts to slip, expect prices to go up accordingly.
Also, "1550" means the hoe can dig about 15.5 feet deep. The JCB numbering system they use generally refers to depth the hoe can dig. "JCB" stands for John Cyril Bamford, a british guy that is generally credited with inventing the tractor loader backhoe. In fact, the name for "backhoe" in UK/Europe is "JCB."
Good luck with your new machine.