Well there's always the standard prices we pay for a massage whether its for 30mins to 1 hr and half. If it's for a massage 30mins is good enough for most ppl so start there, and for an extra go for the cheapest one and you'll be under 80. But do note both massage and extra fits into that 30mins depending on what time the person starts timing the 30mins not you. For the price your asking you can't have both, as time taken to properly massage the back of the whole body head to toe is about 40-50 mins.
You could ask the person to massage for 25mins with 5 mins to spare but your extra in high chance won't be enjoyable and time is money to these ppl. Unless your a premature ejacul that needs only mins this option wont work for you. And the cash you spent for an extra will go to waste.
Trying to save money so you can spend it another time or else where is good but, you have to realise when going the cheapest option wont yield the result your looking for. So increasing that limit till it caters for both needs.