Variable Pricing with Drop Down Menus

I have started using drop down menus in my fillable forms over check boxes but not sure how to get proper calculations with the scripts i have.  Every customer has different pricing and I dont want to have to redo the JS in every contract, or put different values in all of the different drop downs.

Here is what I need: 

Textbox1 - This is the cost of 1 bush trimming (This # changes with every customer) 

Drop Down Menu will have options for:

Spring 

Fall 

Both  (this total obviously needs to be double) 

From here I would like the total from whatever is chosen to calculate in "Textbox2" but ONLY if "checkbox1" is checked. If not then value is $0.00

("checkbox1" is the selection for bush trimming)

Seems it should be simple but not finding the answer. Any Help would be appreciated!!