According to my calculations for a "juice with attitude" would start a pay back @ 32,500 miles. I calculated this based on 11 mpg currently to 12.1 (10% mpg increase with the chip), California diesel fuel cost @ $3.00 gal, and the chip cost of $800.00.
If you need it or want it, the expense could be charged to "pride of ownership". I use that line item to justify the expense for items I buy that are really not necessary.