When you say on demand do you mean someone else is sitting at the computer and needs to initiate the session for you or do you mean you can pull up the desktop whenever you want (unattended access)?
If you just need unattended access then look at ScreenConnect's Remote Access pricing. It should be a little further down on the pricing page.
Access Pricing
Agents Annual Rate
25 - $300
100 - $600
250 - $840
500 - $1,260
750 - $1,716
1000 - $1,920