Like Perun said, those albums were big sellers. This was also before the internet age and people were still buying albums. Sure they weren't popular albums, but people still bought them. Fear of the Dark in particular is actually one of Maiden's more successful albums isn't it? And when Bruce was still in the band the concerts were still huge and they were headlining Donnington and all that good stuff. To my point, when sales started to take a dip in America with Bruce out of the band, they started playing smaller shows.
Is studio time more expensive now? I'm not sure. If you factor in inflation and the fact that album sales aren't what they used to be, I don't get the impression that making an album is a very lucrative process.
Does anyone know the first week sales of some of Maiden's older albums? I was able to find every reunion album (TFF being the highest first week seller at 63,000) but nothing earlier than that.
Fear Of The Dark was bit odd in its success (and lack of). The album was huge across Europe and other markets where it enjoyed sustained chart runs.
But in the UK and USA, the success was much more restrained despite a great start. FOTD made its debut at number 12 on the Billboard chart which was the highest debut a Maiden album had achieved up till that point. However, on its second week, the album fell to number 30 on the Billboard charts and was out of the top 40 the following week and out of the 100 after 9 weeks of release - this was a very short chart run compared to previous albums that comfortably enjoyed longer chart runs, even if not charting higher than FOTD.
In the UK the album made its debut at number 1 but spent only 5 weeks inside the top 75 (less than half the chart run of any prior Maiden album with Bruce on vocals)
Still, the album shipped more than 1 million copies worldwide in its opening week and has probably done around 3.5 worldwide today (educated guess). But, by no means was it one of the bigger sellers from Maiden's catalogue worldwide- the US and UK sales from its first year of release are just too weak compared to that enjoyed by the 80s albums and subsequent back calatlogue sales globally are not as strong as the recognsied classics.
In contrast, TFF has enjoyed much better chart runs than FOTD almost everywhere and it's opening sales of the 63,000 you quote is indeed Maiden's best debut week
but relates only to the USA market. Worldwide, the album shipped 800,000 in its opening weeks. Not that much less than FOTD.
Of course, it has no chance of catching up with the overall sales count of any of the 1980 - 1992 albums, the market is just too weak. But the post-2000 Maiden albums are still comfortably selling at least 1 million per title, a perfectly healthy number.