Memory is tricky. Also, sales prices tend to be a lot more random than MSRP -- somebody always got a whateveritis brand-new for $200 rather than $1000 because the store (had a sale|was going out of business|marked it down for trivial damage|was clearing out dead stock)... There are also regional differences in pricing -- when a store doesn't have much competition they aren't going to mark things down as much, or they can't because it's harder for them to meet the sales minimums required to be an authorized reseller in a small town.You'd have to go by memory. Like, for example, I bought a brand new StingRay 5 from basscentral.com in the early 2000s for a little over a grand.
MSRP is at least a reliable value to track from a consistent single source.