On 2013 use CSR (Client Side Rendering) and a JSLink to override the display of the After Discount field. All the fields from the View are then available in the Ctx object
Wes Preston has written good intros into CSR: http://www.idubbs.com/blog/category/sharepoint/spc14/
Alternative is to use JavaScript client-side, once displayed the same fields are available in the global Javascript variable ctx.ListData.Row
Takes some more programming.
[UPDATE]
I stand corrected.. it actually takes less programming compared to CSR/JSlink
Create a calculated column and paste the Formula, then set the datatype to Number so it executes the HTML
=" <img src=""/_layouts/images/blank.gif"" onload=""{"
&"var TR=this.parentNode.parentNode.parentNode,"
&" item=ctx.ListData.Row[TR.rowIndex-1],"
&" discount=item.Vendor_x003a_Discount,"
&" price=item['Price.'];"
&"console.log(discount,price,item);"
&"this.parentNode.innerHTML=discount*price;"
&"}"">"
The onload function is triggered once the blank image is loaded.
The Table Row has the rowIndex.. but the first table row in the View Table is the Header row
The colon in Vendor:Discount is escaped when the Lookup was created
Price is a currency and a string notation with your locale, but Microsoft provides it as value is well.. only with a weird notation with a dot at the end (which can not be used as valid Javascript, so we access it by the string value
at the end the whol IMG tag is overwritten with the calculation
Only drawback of stuffing HTML/Javascript in a Calculated Column is it will show up as text in Forms (can we hidden) and on Email Alerts
Details at www.ViewMaster365.com/#How