$300 an hour and the average client will probably require 10-15 hours/yr so I'm landing right in the range you outlined.What do you imagine charging for this? I think its perfectly viable to do. I cant tell you how many times insurance clients have picked my brain about their investments or the market or estate issues or other financial questions.
Most traditional fee based planners are charging in the $8k-$10k range for a "financial plan". Which is exactly why most 40 year olds dont have one.
Im not sure what the "sweet spot" is for the fee. Maybe you could do $5K? If you can automate the info gathering aspect of it, then you could probably charge $2500 and it would still make sense.