If I'm OKC, I would approach any trade negotiations for Jackson with the viewpoint that my asking price has been set with the Harden and Waiters trades.
In trading Harden, OKC received two first rounders (one from a lackluster Raptors team that was widely viewed as having the potential to land in the lottery, which it eventually did), the Rockets' lottery pick from that year's draft (Jeremy Lamb), a second rounder, and Kevin Martin. Effectively, OKC traded Harden for three first rounders, one of which was just selected in the lottery and one which had a very good possibility of being in a future lottery, a second rounder, and Kevin Martin.
In trading for Waiters, OKC gave away a mid-late first rounder.
I would assess Jackson's value as being somewhere between Waiters (whom he is currently better than) and Harden (who was better in October 2012 than Jackson currently is). As such, I'd expect OKC to ask for at least one first rounder, as well as probably a second rounder, and one of the Celtics on a rookie-scale contract. I'd also expect them to ask for a draft pick for compensation for Nelson's player option.