As far as pricing, the reason TPM cost more was because it had a lot of the R&D done on that film. Effects-wise it was breaking new ground, and when it came time to do AOTC there was less inventing to do because a lot of it was already there, and they had a ready librbary of plates, animations, etc that they could recycle. There was still plenty of work to do--they made most of their experiments in digital doubling and digital characters--but not as much. TPM also had the most location shooting, with lengthy trips to Tunisia where all the full-scale podracers and significant portions of Mos Espa, the slave quarters, etc. was built. By the time they got to ROTS, effects had advanced so much that it wasn't necessary to do any main-unit location shooting, and there really was no major innovations to make--I think it was said that it was rather the culmination of all their previous efforts.
At the time, TPM was still very expensive, there was still enormous cost-savings because of the way Lucas made in the film and the fact that he owned many of the elements, but $100-$120 million was really the norm for big, effects-driven action blockbusters at the time. I mean the Matrix cost half of that, now that is something impressive, but movies like Godzilla and the like were all in that 100-120 mill range. Lucas' original hope was to do the film for around $70 million, but that wasn't realistic at all. AOTC cost less as I said, but its really not that less--most similar films at the time were in the $150 million range, but were they to attempt what Lucas did they would be much more. I mean The Two Towers has a lot of the same scale and cost much less but much of it was location shooting, while AOTC has a lot of alien things like Coruscant and Geonosis that are entirely false-reality, so I suppose it would be more like a $200 movie had it been made by someone outside Lucasfilm. ROTS is the real breakthrough, because that is more location-based in story than all the other films, yet it cost the same and was all in studio and doesn't look like a PS2 game the way AOTC does.
But in terms of ESB, ESB didn't have that big a budget for its day. I mean it did, but it wasn't unheard of, the reason Lucas made a big stink was because it was his own money. Apocalypse Now cost almost as much, and stuff like The Black Hole and Star Trek: The Movie are getting in the same range. Really, ESB was a fairly realistic budget at roughly $30 million considered the scope and quality of content and compared to contemporary blockbusters of its time. Preportionally, TPM isn't that far off from matching it. What was impressive about the prequels wasn't that they were inexpensive, because they weren't except for ROTS, its what they were able to accomplish for what they paid. TPM was a $150 mil movie made for $120, AOTC a $200 mil movie made for about the same, and ROTS a $250 mil movie made for even less. The problem IMO is that even though thats impressive to Lucas the Businessman, it really shouldn't be to Lucas the Filmmaker, because the effects aren't convincing in most of them. Really to get things to the OT level of realism Lucas needed to actually spend $150 mil on TPM (making it a $200 mil movie made for $150 mil), needed to actually spend $200 mil on AOTC (making it a $300 mil movie made for $200--because thats what Lucas was attempting, and thats why the movie looks like shit). In fact, if he could just control his tech-fetish and shot AOTC on film like anyone who actually knows about imaging systems would tell him (as oppose to Lucas, who wanted to do it simply for the sake of it) then he could have saved millions in cost because thats how much those pieces of shit HD Sony cameras cost him, and that money could have been spent on building better sets or making better CG.