Depends on exactly how you are constructing the reservoir. Are you using the old, out-dated method of progressively smaller rock sizes and gravel or are you using either the matrix blocks or heavy ABS grating?
In either case, I always use pump vaults. They are engineered precisely for this use. I certainly would NOT use a 5 gallon bucket. The plastic will degrade over time and the bucket will collapse. This sounds like something a DIY would try.
I don't see where the cost should be a factor as it is passed on to the customer. I will never sacrifice quality to cut costs. It does not truly give the customer the product that they are paying for and many times it will come back to bite you in the a**.
"Everybody is ignorant, only on different subjects." - Will Rogers