The only soap I've bought so far is a gallon of Chemical Guys Mr. Pink. I think I paid around $28 for it.
Here's the way I look at it:
2oz in a 4gal wash bucket, plus another 3oz in my foam cannon because my PW is a cheapo little electric. That's 5oz per car wash, at 128oz per bottle (not counting any waste, any extra in the foam cannon gets dumped into the wash bucket) that means I get around 26 washes per gallon of soap. That's right around a dollar a wash. Even a crap touchless automatic that will still wreck the finish on my truck is around $8 per wash in my area, so I figure I'm still way ahead.
That of course when I actually do a full foam/soap wash on my truck now because I've been using ONR of late. 1oz:2gal in a pump sprayer and 2oz:4gal in a wash bucket that I use for at least 2 washes, and at $16 per 32oz. it's hard to argue with. At an average of 1.5oz per vehicle, that 32oz gets me 21 washes for $16, or around $0.75 per wash!