I've got an 851 and it's an awesome skimmer, the best I've ever owned. I'm not sure if I'd buy it again given the price, as the price-to-performance ratio is a bit hard to swallow.
The 525 is no longer manufactured if I'm not mistaken. You can call Doug at Deltec USA (877-DELTEC1) and ask him what the MSRP was for it new, and he can probably help you.
A few things to know about Deltecs...
Their customer service is awesome. These skimmers are very top shelf so needless to say Deltec doesn't sell very many... it's like buying a Ferrari, you get red carpet treatment for life as just part of the package. They don't care if you bought it new or used, you can always call them.
Their performance is unreal.
They use usually one of two kinds of pumps... Aquabee or Eheim. Aquabee pumps are practically never seen in the US and finding replacement parts for them is nearly impossible. Eheims are more common, but Deltec heavily modifies them, making many of the parts very expensive... which takes me to my next point...
Spare parts are insanely expensive.. Mine is powered by a Eheim 1260, which is normally a $160 pump. I needed a new needlwheel impeller for it, and it cost me $250 for JUST THE IMPELLER.
So I guess my point is they are an awesome skimmer, but I'm not sure they are so awesome that the price is justified... at least for a new one. These skimmers are in high demand in the used market, so if you find the MSRP, consider anything under 75% of new a very good deal.