calculating product prices via lookup of closest values in table

I want to build an order form where customers can add multiple products and get a calculated price/offer in the end.

The price of some products will depend on the size of the item.
How I need it to work is that the customer enters height and width and that I look up the price in a pricing table after rounding up these entries.

For example:  customer enters 98 wide by 180 high, then the form would need to round that up to the nearest 100, so that would become 100 wide by 200 high and then it needs to use those numbers to find the price in a table and add that price to the total price field.

Any ideas on how to do this?

perhaps the combination of a script and a lookup table? (although some of the tables are pretty large)
thx for your help!