Well, it's worth remembering that, with the Warlords boost, you will effectively be able to buy a 90 anyway, if you're willing to jump through some hoops. You can create a new account and buy another copy of Warlords, along with the previous expansions, if required, perhaps as a battle chest when Mists is inevitably looped into that. Then you've got the $25 cost of a transfer to get that 90 onto your main account. So the cost of a transfer at $25, plus the expansion at around $40, and the battle chest at $5 (at best), the total cost of this method would be $70. Ouch.
It seems that Blizzard would be wise to pitch the cost of a boost to 90 on the store below that. What's more, it seems likely that an $70 price point would be considered far too high by the majority of players. The exercise Blizzard will likely go through in trying to establish a price point for this service would be to ask players "what would you like to pay?" and then "what is the most you'd pay?". Then, shoot for a figure closer to the second number than the first.
My very lightweight research on Twitter has revealed that players would be happy to pay around $25-$40 for a boost to level 90, and to me that seems reasonable. It's considerably cheaper than the $70 we arrived at above, and the cost of a RAF'd 85 would be around $30, with the Battle Chest at $5, with a free month's sub, and $25 for a transfer to get that 85 to your main account.
Of course, if you were to multibox or work with friends you would end up with two 85s with the 300% XP bonus for that money, and there's the grantable levels, so it's actually cheaper than that. Two 85s for $30, so let's say $15 per 85. You don't get all the way to 90 with the grantable levels or the bonus XP, of course. But that's the dollar outlay, the opportunity cost. However, this approach still takes time, so that has to be taken into account. For me, avoiding the time spent leveling, even at speed as far as 85, is worth at least $10-20.
The most expensive character services for solo characters are $30, or local currency equivalent, so it makes sense for level 90 boosts to cost more than that. EverQuest 2 has decided that the appropriate price point for a level 85 is $35, and to be honest that sounds about right to me. That's what I'd be really happy with, and would ensure that Blizzard ended up with at least 70 of my hard-earned dollars. However I fear the price point will turn out to be somewhat higher.
The other option which people mentioned a lot is to offer smaller boosts for a lower cost, and it's an option I like a lot. The suggestion I liked the most was to offer, say, 10-level boosts for $5. This would allow me to skip over the bits of leveling that I dislike the most, so 80-90 and 1-20 for example. I'm not including this in the poll below, as it's not something that has been mentioned by Blizzard at any point.
The question is, then, what number do you feel is right for a boost to 90 for any character?