You'll spend more than that
You'll spend more than that for a Chevy S10, and remember your truck weighs a lot more than a little Opel GT. So don't expect that kind of mileage, even in an electric hybrid conversion.
Dollar for dollar, my suggestion is a water hybrid. Not sure if you have a 4 or 6 banger in there, but you could breathe some new life into it for just a couple hundred bucks. Water Hybrid Power is all the rage and it's sweeping around the world (our hybrid water conversion books sell in over 200 countries).
Essentially, you use simple electrolysis (high school chemistry) and your cars' alternator to make on demand hydrogen/oxygen which is then injected into your car through the air filter ducting. It mixes with your normal air/fuel and makes it combust more efficiently, giving better mileage AND cleaner emissions (by over 50%).
It would also clean the years of sludge and build up out of the inside of your engine.
30%-50% increases in mileage are not uncommon.
That's how I'd go...save money and make the conversion in a weekend
Build your own Hydrogen Generator
Well I don't know anything
Well I don't know anything about diy and transforming my car,but I was looking to buy a Prius Hybrid Toyota S 2009 Sedan from an used cars Columbus Ohio dealership.I'll guess my son will do some changes to it,but not me.