The 840 EVO is over 2 years old at this point (1st quarter 2014). Back then that's what a top of the line 500GB drive cost. The 850 EVO came out and was about $100 cheaper. So probably what you see is that because the 850 EVO is their current active model, the pricing stays competitive. The 840 though is old and probably forgotten about on retailers websites, so they haven't updated the pricing. I would assume you are talking about Newegg's site?
You also have to consider that the retailer purchased the 840 drive for a quite a bit more at that time compared to what they bought the 850 for. So from a customer point of view you look at it and say why would I buy an older product at a higher price? But the retailer paid more for the 840 then they did the 850. So they can either sell it at a loss or just keep it hanging around in storage hoping someone will bite at the higher price. Personally, I'd wait it out if I was a retailer.
EDIT: I did buy an 840 EVO 120GB at Fry's on a flash sale for like $60 a couple months ago. I put it in a USB 3 enclosure and use it to carry around bigger files. So there are deals on them.
Supply and demand at work: people want the 850 series products because they're newer, more efficient, and obviously higher performing at this point in time and they're costing Samsung less money to produce them so they can price them at lower retail points than the older more expensive to produce and less in demand 840 series. westrock2000 covered it pretty well in the post above, basically.