Too high. It is a nice bike but it is old. The Shimano 600 basic design is the same as the current Shimano 105 with one big difference, the 600 rear derailleur in 1992 was a 7-speed derailleur. The current Shimano 105 rear derailleur is a 10-speed derailleur. The 600 RD does not have enough swing to accommodate an 8-speed cassette so you cannot upgrade it.
The rubber hoods on the brakes look to be in pretty good shape but it you ever have to replace them, you will be pretty much SOL unless you are lucky enough to find a pair on eBay. I believe that they were still using freewheels in 1992 rather than cassettes. Finding a 7-speed freewheel could be an ordeal. If it is a 7-speed cassette, good luck finding one. When they do show up on the Internet, they are usually extremely expensive because they are so rare.
One last thing to consider: there is a rule of thumb that pristine bikes tend to lose 25% of their value in the first year after they were purchased, and then they lose 5% every year after for the next 4 years. After that they lose 10% - 15% depending upon how outmoded their components are and their overall condition. This is assuming that they did not have serious design flaws or recalls and that the bike is in good serviceable condition. The Cannondale SR800 had an MSRP of $2200 which means that if it were perfect, it would still only have a value of $330. Unlike automobiles, bikes do not reach a classic stage and then an antique stage when they begin to appreciate in value. There are only a few collectors of antique bikes world wide and precious few museums.