I think "a lot" is too light for just how much of a gigantic fucking mess the whole thing was, from the top of my head, they wanted the falcon missiles instead of the 530 (even though they were roughly the same), so that needed a new radar, but the IIIC was too small, so they needed a IIIE, and that came with a new engine, the atar9C, more powerful but more expensive.

They had to modify the landing gears so the IIIS could fit in those bunker caves, they also wanted air to ground capibilities on the IIIS (for some reason), so they added hard points on the wings and the ability to fire the A30 nord missile, which means the IIIS had THREE joysticks in the cockpit, one for flying, one for the radar and one for the A30 guidance.

And that's before they had to fit the TARAN system in the mirage IIIS which IIRC was a massive pain in the ass. Caused the budget to balloon by 66% of its base cost, which caused a giant shitstorm, chief of staff and federal army supervisor resign and the cmder of the air troops is fired, yeowch.

In the end they only bought 56 of em, and only 30 or so fighters out of a planned 100... yikes. I've read some of em actually collided with trees but managed to limp back to base and allegedly some pulled off 12g maneouvers to avoid cliffs and mountains without damage. Just an insane environment to fly a 1950's delta wing in.