Fuel Economy: Is a Toyota Prius C worth it?

Gas prices are going up again and the end is not in sight.  Gas prices are a mystery, differing from state to state and fluctuating week to week.  As one who commutes 50 miles a day, this tends to be something I think about often.  I decided to do a little number crunching to see what the cost difference would be between driving my 1999 Lexus GS 300 vs buying a brand new 2012 Toyota Prius C.  See the comparison of the two below, compliments of www.fueleconomy.gov.

2012 Toyota Prius C

So here are the numbers I calculated.
The MSRP of a new Prius C Two is $20,000 (including a $660 haggling discount).  This is a payment of roughly $400 a month.
Over 5 years that is $24,000.
Fuel for 1 year is $1,150 and 5 years is $5,750.
If the resell value of the vehicle mimics a 5 year old Toyota Prius today, it would sell for somewhere between $12,000-$15,000.  Let's go with $14,000.
So, the cost to drive a Prius C for 5 years is $24,000+$5,750-$14,000 = $15,750.
Per year, that is a cost of $3,150.
Per month, that is a cost of $262.50.

1999 Lexus GS 300

Now here is the cost of driving my 1999 Lexus GS 300.  My car is paid for so the purchase price is $0.
Fuel for 1 year is $3,050 and 5 years is $15,250.
If the resell value of my vehicle mimics current 18 year old Lexus vehicles, it would be somewhere between $1,000-$2,500.  We'll say $1,750.
Now, since the car is coming up on 200,000 miles, we need to factor in repair costs.  Over 5 years, I'm estimating $1,300 in repair costs.
So, the cost to drive the Lexus GS 300 for 5 years is $15,250 +$1,300-$1,750=$14,800.
Per year, that is a cost of $2,960.
Per month, that is a cost of $246.66.

The additional cost per month to drive the Prius would be $15.84.  I'm surprised at how little of a difference there is between the two choices.  $15 is marginal, money spent in a heartbeat at lunch out at Cafe Rio.  Now I understand that these numbers are not cold, hard facts.  Some of them may be wrong.  I didn't do a full, lengthy analysis on this, so I may be missing some other essential elements.  I didn't take into account other maintenance costs.  Also, if my Lexus were to blow a head gasket, have the transmission go out, or die altogether, then this numbers would also need to be adjusted.

So the question is, do I want to drive a Prius or my Lexus?  It should be noted that I have hated the Toyota Prius with so much zeal that it is amazing I actually 1)wrote this post and 2)am even considering the Prius as a car worthy of buying.
If you see errors with my calculations, please let me know in the comments section.  Remember, it was my brother who received the Math Sterling Scholar award in high school, not me.