You can assume whatever you want. To my knowledge, most private schools won't match subsidized IS tuition at public schools, which is why many people end up at IS public schools! 🙂
Moreover, why do you think IS isn't an all or nothing proposition? You cannot establish residency in most states while a full time student, so, as far as I know, if you begin as OOS you will end as OOS. Where did you hear "declaring IS residency is pretty easy"? I heard exactly the opposite.
Most schools are very strict about it, and it is typically determined by rules established by the state, not the school, because it is the state subsidizing you if you are IS. If you have been out of school working for at least a year, you are IS where you live and work, not wherever you "declare." And, if you are not independent and don't provide half of your own support, and your parents declare you as a dependent on their tax return, then you are IS where they live and work. Again, not wherever you "declare."
To the extent people are successful negotiating price discounts, it is almost always getting a school to match a grant (either merit or need based) offered by a school it considers a peer (i.e., T20 to T20, T50 to T50, etc.). Hopkins won't match Drexel, but might match Penn, and will never match UMD. JMHO. You can certainly try anything, but it will be useful if you go in with realistic expectations. Good luck.