This extension does three things calculate price by formula , change product price on fly and manage dependent options in the price of one extension.
It will automatically update price when you select the options, even if your product is not using formula.
One can write conditional / dependent options using this variable.

Dependent Variables are available from opencart 2.3.0.2 version

For version 4 , it works with default opencart structure only and making changes to your theme may take extra. Please make enquiry before buying for version 4.
Very often you need your product price to be decided by some formula like length or area or volume .Here comes the rescue , here you can have dynamic product price driven by variables , you can write formula using these variables.

Developed by shambhal who holds maximum opencart reviews on freelancer.com
This extension is vqmod for <2 opencart and ocmod based for version 2 onwards, therefore no core changes
You can also set constraint of variables max and min value if(option type is textbox) , like for length you can have l:100-450 so it can accept any numerical value between the range.
Since 2.3.2 version -version 3.x.x. it is compatible with journal2 , journal3 theme but developer holds responsibility for default theme only.

Variables are assigned to options so you can have better control on options.
It works with options in select combo,radio and textbox

sample formula is
It helps you have dynamic price calculation on frontend , plus it has feature to have advance options.

Please note that it works on default opencart structure ,so if you have any extension which contradicts it ,it won't work.
so please confirm before buying .
demo
Backend see product with product id 35, name =formula test
http://formuladep.opencartext.com/admin/

formulatest/123456 username/password
<br/>
Frontend
Formula with controls
http://formuladep.opencartext.com/index.php?route=product/product&product_id=50
Area Calculation
http://formuladep.opencartext.com/index.php?route=product/product&product_id=47
Formula extended
http://formuladep.opencartext.com/index.php?route=product/product&product_id=30
Dependent Options
http://formuladep.opencartext.com/index.php?route=product/product&product_id=28
Dependent Options and Formula combined
http://formuladep.opencartext.com/index.php?route=product/product&product_id=51
Constraints on variables assigned to textbox can have numerical values only.

There is another extension available if you want price to be determined by some function
http://www.opencart.com/index.php?route=extension/extension/info&extension_id=23399
Currently supported themes
Default opencart, Journal2,Journal3
if your theme is custom we could make extension compatible with your theme on little price or free.

Extension bought once will be valid for the domain and its 5 subdomains.
For further queries you can ask me at shambhalnetworks@gmail.com