It certainly should not be more expensive in the fall/winter, so to the extent there is any seasonality in pricing, the fall/winter would be when you benefit. Having gone the O/B route in spring/summer, I can attest that (i) you can save money going O/B instead of PB, but (ii) when a contractor is busy, they are much less interested in negotiating on price.
I wouldn't expect to receive many discounts just for showing up during the off-season. But if you are willing to get multiple competitive quotes from different pool builders (which you should no matter what season you are in) and willing to try to squeeze their margins by working them over, then I think there is a potential for non-trivial savings in the off-season.
"Attached spas are just areas for adults to socialize in while the kids play in the pool." - This may be mostly true during the summer, but in fall/winter/spring, the spa (with some heating) becomes the main pool for my kids to really extend the pool season. Mine are 3-10 yrs, and will spend hours in the heated spa with breaks to take polar bear plunges into the pool. That's not to say that a stand-alone spa wouldn't do the same and at a cheaper price with more therapeutic options.