Setting this up correctly in DataAssist Search you will need to have two criteria. One for the start and one for the end. For the first criteria you will select the price column, then for the type select Number, then for the comparison you will select less than. Finally for the type choose value, and for the value use the lightning bolt to select your min price field.
For the max price you would do the same thing but choose greater than for the comparison and select the max price input for the value.
What are the steps you have taken and what is the result you are getting?