Here in Reno at VP Racing, 100 Octane historically had two prices $4.99 and $5.99, I'm quite glad to see it at $5.99 once again as it was up around $8.
The best you'll get out of most octane boosters is about half to one point, which is generally a waste of money as your engine will never know the difference.
What is your engine running that you need 100 Octane? Maybe detune it a little and make it more street friendly? Consider buying a 55 gallon drum of 100 octane direct from VP to bring the price down if you have to use it all the time.