It's one of the biggest jumps in price from one view category to another in all of disneyworld -- about $100 the time of year you're going. (Just by comparison, the GF is about $60 to jump from garden to lagoon, and I think it's pretty much the same kind of jump.)
I think there's a double message there. On the one hand, it's a considerable jump in price and thus makes it a tough decision. On the other, it tells you that people really like the lagoon view and there's a good market for it, which suggests it's much better.
I think one big difference between lagoon view at the Poly and the various water views at other resorts, is that there are virtually no bad lagoon views at the Poly. Going by tikiman's maps, I'd say maybe there are 4 questionable views tops. In all the other resorts, you can pay for the better view and be a little disappointed -- the Beach and Yacht and BWI for example. Even at the GF, there are maybe 10 percent of the lagoon view rooms that aren't really worth the extra money. Not so at the Poly. (Again, of course, I haven't stayed at all of them -- just going by tikiman's maps and from having been on the grounds.)
My bottom line is that the poly garden view, with some discount (e.g.,
AAA) is perhaps the best value in all of
disneyland, taking all things into account and assuming an intention to spend some time at the MK. Maybe the non-tower rooms at the contemporary would have a very slight advantage. On the other hand, the lagoon views at the poly start to lose an awful lot of that value -- there's a lot you can do at disney if you're prepared to pay for a $410 rack room. For a long stay, it starts to add up.
Long winded post that manages to say very little, I guess, but ultimately, if having the lagoon view is something you would think of as "nice" but not cruicial, save the cash. If what gets you excited when you think about the trip is primarily attractions or shows or meals, then it's probably too much. But if what gets you excited about the trip is your resort, and your view, then it is pretty great.