Well I disagree. Trying to bring the opportunity cost of Frolik is really only relevant to the value/analysis of Stafford's UFA signing, but it becomes far too removed from the trade to be a factor. If you want to value re-signing Frolik, then you also need to look at the UFA market and who we could have otherwise signed instead of either of those two (or even available trades).
However factoring in Stafford performance from being re-signed months after the trade is really not a stretch, and not an overvaluation in the sense that it is related to the big event. Again, I'm not advocating allocating every goal he scores as value of the trade. But, if this contract ends up being very successful, it does add to the positive effect of the trade. If he ends up being a contract bust, then it does remove some value on the trade. Either way it's not a significant valuation factor, but it is measurable. I'd say we'd need to at least see a full year of Stafford's contract before casting judgement on it however, unlike what we got from him on the balance of his Sabres contract (also with the reduced cash salary hit).