Question about the price of 340s

Hey guys I have been workin on my 70 Dart swinger 340. I have noticed people on moparts and ebay and criagslist selling 340 block or complete engines. My question is why are people asking so much for them. I bought a complete 440 engine for 1000 but see people asking for twice that much for a 340? Is it just that they are scarce?