I charge my normal hourly rate for year end tax planning/projections.
W2 people don't really need tax planning usually.
Self employed and S corp clients/partnerships are your primary target area.
Tax about where they are for the year, how much they might owe based on how things currently stand, talk about equipment purchases, depreciation/179, or retirement options that might be able to save them tax, figure the expense or retirement, calculate the new net profit and the new taxes.
If you can show them how much money you are saving them in taxes, it gets easier to charge them.
If you spend a bunch of hours and can't save them anything in taxes other than to avoid the underpayment penalty, you aren't really providing them much visible value, which makes it hard to sell a decent invoice for your time invested.
After October 15th deadline is a good time to go through the files, identify the clients that could benefit from your services.
Start contact them in mid-late October for Nov-Dec tax planning. You want to give them at least 2-3 weeks to arrange financing to purchase equipment, etc. Good way to keep a little revenue rolling in before next tax season hits as well.