Worth is relative. We have no starting DTs besides Mazi. $20 million is the going rate, so pay him that for one season and try to draft his replacement. The only other option, at least for this front office, is to bring in some scrub as a fill-in and draft his replacement, knowing that you not only absolutely have to hit on that player, but do it immediately his rookie year, even though most rookie DTs don't put up top numbers as a rookie.
Most likely, this front office will give him a four-year deal at market value. I'm just saying what I would do. If we were going to go after someone like Milton Williams if we let Osa walk in free agency, then I'd be good with that, but I absolutely do not expect that to happen.
It's easy to say just draft his replacement, but 1) we've seen that first-round picks for DL don't always work out and 2) the draft may not fall in such a way that DT is the best option when we pick (we can try trading down, but you have to have a partner).