It might be cheaper to buy as single bottle of 0.5% Captain Jack's, but it's only gonna make a tiny more than one gallon. Don't even think about the ready to use stuff, it's not concentrated enough.
To make one gallon using the 0.5% concentrate to need to use 27 ounces of the 0.5% concentrate and 101 ounces of water.
Elector PSP made gallon
1 ml of Elector = 442 mg
Directions say ~9 ml per gallon
9 ml Elector + 3776 ml water = 3785 ml (one gallon)
9 ml per gallon x 442 mg per ml = 3978 mg spinosad per gallon
I think that works out to a dilution of ~0.1% spinosad per gallon.
The above should be the reference when trying to make other spinosad sprays. Target amount of spinosad per one gallon is 3978 mg or 0.1%
Captain Jack's 0.5%
3978 / 5 = 795.6 ml (27 ounces) needed to make one gallon
1 ml of Captain Jack's= 5 mg
795.6 ml per gallon + 2989.4 ml water = 3785 ml one gallon
795.6 ml per gallon x 5 mg per ml = 3978 mg spinosad per one gallon