the last post has a good point. this happen a lot in the minds of buyers. To be honest, I would just sell it at a 15-20% discount. That is the local discount rate that many bookstores carry. You can say this is the author discount. Since there books are signed and personalized, they ARE collectors items in the mind of most readers.
In bookstores, signed copies often mean that a person is willing to pony-up the extra cash for a HB. When I worked at Waldens, I could easily sell a signed HB over and unsigned PB.
Another point to consider is the number of copies you have. Its only 120, which means you didnt invest a ton to get them. I would calulate a break-even point at different prices. I mean, really, its not difficult to sell 120 signed/personalized copies of a book. And if they sell slower, so what? I don't think your next months mortgage payment is relying on these books.
If readers want a premium product, they pay a premium price. Take, for example PS Publishing -
http://www.pspublishing.co.uk/ - now, im not saying you should sell you books for the price these guys are, but its a good example of how people are willing to pay for collectors/signed editions of novels. By providing the fans with a method of gaining a signed copy, you ARE helping the fans. You dont have to sell the books at cost to achieve this.
As far as bookplates go, I think you should offer it once your 120 HBs are sold out.