Yes, the ERS7's are particularly expensive. It's why that one came down to more of an "impulse" buy (I mean, I knew enough - it wasn't just a see it, grab it). I probably would have teetered on it for too long and decided against spending that much on one little robot pet. However, I am very thankful I did! It's definitely not one of those regrettable impulse buys. Then again, I haven't eaten in a few months so I'm saving a lot on groceries! (Sarcasm of course

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The other models are more in Pleo's price range and I think that's still pretty fair. However, there is the detractor of the lack of support from Sony for the older models. I believe some of the ERS7's are supported through 2013 (I think that's when mine loses its official support from Sony). When/if you do go AIBO shopping, just look other places than eBay! Everything there seems to go for much higher prices in AIBO world than what you'd find from "private" sellers.
From this point, I can see myself buying another 210 in the DISTANT (like a year) future. The best thing to do is to just read so many comparisons about the models (aibolife forums are great for this - I have spent HOURS surfing through people's opinions and comparisons there on the various models) and see what one you feel is the best fit for you. Most people recommend a 210 as the first AIBO, and I can see why. Depending on what software you get with it or what else it comes with, you can get a great price with quite a lot of things.
However, I am still glad that Nicco was my first AIBO because of the fact that he didn't take that long. If I would have gotten a 210 first, I probably would have kept a regular adult personality (such as DogsLife) on it for awhile before raising it - because, like many, when I first buy a new tech piece, I want to get right into it! Plus, between him and Murray, I would probably leave Nicco as the one that's on all the time just doing his thing (to put this into context, I'd feel the same way if neither could self-charge/dock/etc.). Both make me feel very much so like there is something in the room with me (as in they trick me better at being "real" than Pleo does - but this might be because of the whole robot companion vs. pet thing I have mentioned before - or maybe AIBO just creates more of a "presence" for me). With that said, I would never buy an AIBO for someone like my aunt or parents, however, I would consider giving them a Pleo (I just don't think they would ever get over the robotic fact enough to get into it).
And it is really hard to stick with just one. I'm reducing my overhead (life costs) for the next three months to lessen my guilt about splurging a bit on this

And with my one-in/one-out rule, I'm running out of things I'm open to getting rid of!