There are 2 issues here - Kindle as hardware and Kindle as content. For Kindle as hardware, I would not pay more than the 60$ I paid for my Sony with several books of my choice thrown in as Sony did; for me eink has been a major bust until now and unless things change (speed, touch screen, color) Kindle just does not interest me.
For Kindle as content, it's hard to say if a subscription model would tempt me; I guess it would depend on what's in and how much it is.
I had unlimited Rhapsody for several years for 10$ a month (Cablevision offered that for their broadband customers - do not know if the price was general). For various reasons (incompatibility with iPod, most artists I am interested in were not available) I did not use it that much, but for 10$ a month, occasional use was ok. When they increased the price to 14$, I canceled; so the same would be true for a book subscription - at a reasonable price and with enough tempting books, I would go for it, and then it would depend on usage...