If the ban is lifted what can i do to my post ban gun? Can i put a flash suppressor, bayonet lug, or a collapisble stock on my rifle? Thanks.

Yes.

Also you can buy new hi-cap mags.

Also why are post ban guns almost as high as pre ban? Cause ive seen pre ban go around 800-1000

There could be a lot of reasons

Not a real pre-ban, but made up to look like one.

"motivated" seller.

Someone is selling it, as a used AR-15, not knowing the whole pre- post- thing.

Stolen.

Estate sale.

Lots of reasons, some legit, some not.

There are many rifles, post and preban, in many different configurations, by many different manufacturers in many different states of use. Pre-ban values are pretty much what you can sell them for right now, or what they are worth to their individual owners, with rarity and the AWB factored in, multiplied by fear of the AWB not expiring and divided by fear that the AWB will expire and cause a drop in value. Add the "gun show " variable in, and you could end up with damn near any asking price.

So your question really has no answer as far as I know...