We often rationalize large-ticket purchases based on price, and even more so when our pocketbooks are pinched. We see a lot of hybrid affordability comparisons these days. Here’s mine. So, let’s do the comparison.

The manufacturer suggested retail price (MSRP) of $22,000 for the all new third-generation 2010 mid-size Toyota Prius II is $1,806 less than the $23,806 median price of the mid-size sedan segment.

Compared to vehicles in its segment, the compact class Honda Insight LX (base model) is priced $1,400 higher (2009 compact car median price is $18,395) and has only a modest fuel-economy advantage.

Let’s do the math based on a Prius II (base MSRP of $22,000) versus an Insight EX (base MSRP of $21,300). These models are equipped the way most consumers want.

Using industry loan averages (60 month term, 7.45 percent annual percentage rate [APR], $2,400 down, tax, title and registration), the Prius has a monthly payment of $435 while the Insight carries a monthly payment of $420…a difference of $15. That’s just a couple of Starbucks double shot Grande Lattes a month.

Now let’s factor in annual fuel costs based on the EPA-estimated fuel economy rating of 50 mpg (city and highway combined) for Prius and 41 mpg (city and highway combined) for Insight and 15,000 miles at $2.62 a gallon (national average as of 6/8/2009). The Prius will save you about $172 per year or $14 a month over the Insight.

Now the overall difference is just a little over a dollar. So for a buck more each month, you get a roomier vehicle that is a full hybrid with better mileage ratings, enhanced performance, vehicle stability control, driver’s knee airbag, beltless engine, rear disc brakes….well, you get the drift.

If you compare the Prius I (base MSRP of $21,000), due out later this year, to the Insight LX (base MSRP of $19,800), which is still not an apples to apples (size and equipment) comparison, the monthly payment difference including the fuel savings is only about $12.