I can see where this is going. You're going to have different price ranges that you want rounding differently. You can set up a table thus:
then use it like this:
=ROUNDDOWN(G3,INDEX($L$2:$L$9,MATCH(G3,$K$2:$K$9)))
where G3 contains your original unrounded price. Copy it down the whole price list.
It's up to you to tweak it, I'm just showing the principle.
See attached.