If you want a sense of how much used TiVos go for, you should search around on ebay. I think TiVo HDs with lifetime generally go for around $150-$250. Ebay, Craigslist, and the Amazon Marketplace are the most common places that used TiVos are sold. You can also check out the Buyer/Seller Area on this forum:
When you call to disconnect service for the Premiere, see if TiVo will give you the $99 lifetime deal on it. If they do offer it to you, take it. The resale value of the Premiere will be much better with lifetime.