how much will this cost me

i need a sub box

box tuned to 35 hertz, two 4x18 inch tubes made with like 5/8 or 3/4 md board

i'm thinkin like 180, but what would you guys expect to pay for a custom box like that

You intend this to be a ported box or a bandpass box?  What size/type/model# subwoofer will go in it?  And  what do you want the internal volume of the enclosure to be?

You could probably build one for less than 50 bucks if you have the required tools and know-how.  This is NOT an easy task, even for someone who's good working with wood.  Even more difficult when you're talking about ported and bandpass boxes.  Unless you're talking about a custom sub box such that it's shape is not a traditional shape, you could easily find a box like this for less than 100 bucks.  Certainly less than 180.