It's almost always cheaper to fix than replace a car. However when faced with major repairs I always recommend two things.
- Second opinion/estimate
- Cost analysis
Get a second opinion and estimate. In your case base on the information provided the estimate is really high. I am assuming the "heatbox" is the Heater Core Case if so that's about a 5 hour job, not 10, if heat box means the heater core or heat exchanger it's even less time.
Once you are happy with the recommended repair and the estimate do the following cost analysis.
If what you can sell the car for as is, plus the cost of the repair is more that what the car is worth fixed. Then it's not worth fixing, you lose money.
Sold as is it's worth $2000
If fixed it's worth $3000
Cost to fix is $1700
If you sell the car you get $2000
If you fix the car you have an asset that's worth $2000 and you put an additional $1700 in it for a total of $3700.
In that example you would be putting $3700 in something only worth $3000 so you loose $700 by repairing the car.
If the repair cost is $700 it would look like this.
Car is worth $2000 as is, plus $700 for the repair, for a total of $2700. The car is worth (fixed) $3000.
In this example you have a total of $2700 in something worth $3000 so you gain $300 by repairing the car.