The big reason for the price difference deals with the molds. Injected plastic kits (whether made by DML, Italeri, Tamiya, or anyone else) use a mold made of some form of metal (different companies use different metals). These molds can be used thousands of times before they become worn enough to require replacement. Resin kits on the other hand, are typically made from a silicon rubber (or similar material) that does not last nearly as long (maybe one hundred or so times) before it needs replaced. Resin is also a more expensive material to begin with. I have seen very good kits made in both medias (resin and plastic), but in the end, if you want the detail associated with the resin you will also be stuck with the higher price.
I hope this helped shed some light.