Conversion from carbs to Megasquirt Efi would cost around 1000 euros. Depending very much from how much you can do it yourself and what price you can get injection intake manifold and other parts.
You will need at least following parts:
Intake manifold, Volvo part 1000552
Fuel injectors 4pcs
Fuel rail
Throttle body for Intake Manifold
Fuel pressure regulator
Throttle cable
Injection wiring harness.
Those mentioned above is best to buy in one piece, from junkyard GLT for example.
Fuelpump from fuel injection car, I buy a new one, but I think that used will do.
Megasquirt kit, that includes also map sensor
Wires, lot of
You can use existing cylinder head temp sensor, software need just little adjustment.
Intake air temp sensor.
Throttle position sensor, one from Volvo 850 10valve should be ok.
Then you need to mod carb version cylinder head a bit, into carb has coolant circulation, you need to block these holes from cylinder head. I think this is hardest part of project, I will buy this service from company that has experience from engine works.
Optional
Lambda sensor, recommended though
I hope that I did not forgot something here 'sm14'
Then of course little bit of welding, small tip welding device recommended and also some wiring needed to car.
I'm not very familiar with cars tech side, either with electronics, but I take this project as lesson into new world. I think that I have 50-60% chance to make it to work
Here is some more information that I found interesting:
http://1800philes.com/tuning_article.html