Nice bike, but as far as desirability (and thus price) are concerned, it's not in the same strata as something like a high-end Peugeot PX-10. I'll let someone else help you put a dollar amount on the bike, but to help them out, could you tell us where you're located?
As for trading it for something indexed "like a Schwinn"... Schwinn and Peugeot both sold bikes at widely varying price/quality points, and they both sold bikes with indexed shifters as they were introduced and the market went that way. Much like any other bike maker at the time. If you do wind up trading your Peugeot for something like a Schwinn, just make sure you do it for (at least) a comparable-level bike.
If you like how your Peugeot rides besides its shifting, another possibility is to upgrade its drivetrain instead of selling or trading it. Having a 6-speed freewheel, there's a good chance your frame's rear dropouts are spaced at 126 mm. You could potentially swap your 6-speed freewheel for a 7-speed. Pick up some indexed shifters and compatible derailleurs and you're in business. If your frame has the funky one-sided Simplex downtube shifter boss, that might be a hurdle, but the swap is otherwise straightforward and not terribly expensive. Heck, you could even swap the downtube shifters for modern brifters if you're so inclined.