If it's the expansion bay compatible ones you're interested in, you're limiting yourself to only a handful of compatible devices. If you don't use rack mounted devices, and I don't think you do, the only available expansion bays are the DX513 and the DX1211. The DX513 is compatible with 3 NAS units, the DX1211 with two. All the compatible NAS servers are from the "+" series, the most powerful series Synology has to offer (so no problems in the CPU department). Of course, this also means these are the most expensive units.

Eg. (I'll use Euro prices here, but I suspect the situation in dollars will be the same) The DX513 is the cheapest expansion bay and costs about 530 euro. It can be used with the DS712+, the DS1512+ and the DS1812+ NAS units, which cost 410 euro, 700 euro and 840 euro respectively. This means that he cheapest unit, the DS712+, which is only a two-bay unit, combined with the DX513 will cost you 940 euro for 7 bays. This, while a DS1812+ would have only costed you 840 euro, AND would have given you 8 free bays! Not a cost-effective option if you ask me... If you look are the comparison table, you'll notice all those units are from the "Small and medium business range".

In my opinion, the best price/performance offering currently (for home use) is the DS413 unit. It's 4-bay, quite fast and costs about 450 euro. But it cannot be expanded.

To be honest, I find those expansion bays quite expensive by themselves - I think I'd rather invest in a big NAS with a large number of free bays, instead of buying an extra -expensive- expansion bay to go with it. You'll probably be cheaper off in the end anyway. And, if I'm not mistaken, you also cannot combine the disks in the expansion bay and the ones in the main unit into one big volume. You'll need to make (at least) two volumes. This defies the purpose if you ask me.