I'[m new to the whole building of teh bass, and im still doing research on what parts to get, wood, pickups etc. but I had a question on the what different bridges do. Some are like 150 dollars while others are 30. Is there a change in something?
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Better bridges will sustain better and offer options such as through body stringing. Besides that, many will also offer better action and intonation adjustments than inferior bridges.

Bottom line, don't skimp on the bridge.
The bridge is what transfers the vibration of the strings to the body so really don't cheap out on it. Like IRS said, a better bridge will offer better tonal options and depending on size/mass, greater sustain. There will also be much much better quality workmanship than on cheaper bridges.
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Bridges can also have an effect on the tone. A heavy brass bridge will give a much heavier low end than an aluminium bridge, which will give a briht punchy tone.
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There is some good news also. Many bassists are not at all looking for an agressive, punchy and sustainfull sound with stratosferic overtone as offered by expensive cast or milled bridges. Instead they want a more rounded and mellow sound with more down to earth overtones. These people are best served by traditional old school vintage style bridges, which are basically angled pieces of sheet steel with a few holes drilled through them. Cheap but effective. If you make one yourself, and use stainless steel to skip the chroming, they cost a few cents a piece.