I'd decline at that price, personally....even if it did have CV/gate. CV/Gate is really just an old school way of controlling synths/etc with an external device. However having CV/Gate makes it less of a job to MIDIfy a synth, as there are off the shelf adapters available that won't require you to modify the synth internally, just plug and play more or less. Its not impossible to add MIDI to one without CV/Gate, but definitely a bit more complicated. I am not providing this service... but I would assume you're looking at a ballpark of at least $300+ for a tech to add MIDI, depending on if you find someone local to do it or have to ship it. Despite the synth being in amazing (physical?) condition the synth could easily have all kinds of things wrong internally, if the synth has not been recently serviced I would assume that it needs some TLC at least...its a pretty old synth really...not to turn you off of vintage synths just a reality, and I have bought dozens over the years.
For accurate price references, always check eBay...and click "Sold" listings...then you can see what they have sold for in the last 90 days or so. For example, here is a "fully functioning awesome sounding" Moog Prodigy without CV Gate that sold for $760. That is the most recent one, but it is also an above average price for this model as well...
http://www.ebay.com/itm/Moog-Prodigy-Sy ... 3381970be7
Here is one "fully restored" without CV that sold for $810.
http://www.ebay.com/itm/moog-prodigy-sy ... 4ac2ed237c
Now I'm a negotiator on these type of deals, so if the seller can provide documented proof of the Prodigy recently being serviced, and be able to demonstrate its full functionality, and I was in the market... I would pay $600 absolute max, in reality probably less...I'd certainly start the bargaining in the low $500 range. A few reasons:
1. In comparing eBay prices, these prices are always going to be higher than "in person" transactions, because its a global market and everyone can look at it. eBay however is very much in favor in the buyer's experience. A seller will have to pay 12% of the final item value in fees to eBay. THEN, they have to pack the item securely and hope it arrives safe. THEN, if the item does not live up to the auction listing...the buyer can send it back for a refund. So really a lot of hassle for the seller to use eBay. Consequently, as a buyer you pay a little more for that extra assurance that the item will be as described...
2. Not having CV/Gate is automatically going to put this Prodigy at the bottom of the desirability scale of Prodigys...and consequently closer to the bottom of the price range than your seller thinks. It will be expensive and maybe even a royal PITA to get it to interface with MIDI. If MIDI is important to you I might suggest you hold out for a different Prodigy if that's what you want.
3. Its a very old synth and is likely to have a few problems, even with a verified service history.
4. There are modern made analog synths out or coming out soon that have "comparable" features but with modern conveniences like MIDI for similar money...such as the Moog Sub Phatty, Korg MS20 Mini, Novation Bassstation 2...and of course the advantage of not being 30 some years old already. Now the sound may not be the same...but...I guess it depends on what you want most out of a synth. If you want something that you can play live with, then I would absolutely consider a modern analog over a vintage.