I guess the question would be whether or not a very good PF is really someone who comes 'cheap' today? Players like Boozer, I believe, get $14M per year.
Realize, although Larry's the man, he'll be joining a team with a pretty fluid offense but may have some holes in the 4 spot or on the wings, in terms of mainly defense. If the assumption here is a type of -flat tax- on the ancient Celtic, then the savings by bringing in a top tier PF, like Cowens, could then be applied on getting perimeter shooters or defensive stoppers on the wings like the next James Posey or Bruce Bowen.
I know it's heresy to say it, but I think you might be overrating defense, at least as compared to elite offense.
The Celtics were the #18 offense in the NBA last year. If I was going to plug a hole on the team that had Howard, Rondo, and an aging Pierce, it would be on offense, not D. We lost in 2010 because we couldn't consistently score, and we probably lost last year for the same reason.
Our defense would be fine. Last season, the Magic were the #3 defense, despite starting poor defenders like Jameer Nelson, Hedo Turkuglu, Ryan Anderson, and Jason Richardson, with Gilbert Arenas seeing heavy minutes. Think about that. Dwight Howard + 4 bad defenders = excellent defense. Replace Nelson with Rondo, Hedo with Bird, and Jason Richardson with Pierce, and you have an even better defense.
With Cowens at PF, we'd have a better defense, but perhaps an even worse offense; Cowens was never a terribly efficient player. As we saw last year, great defense + mediocre-to-bad offense just isn't good enough to win in today's NBA.