Matthew McCallum wrote:
The whole Marvel line sold approximately 7.5 million copies per month in 1987. Leaving X-Men 1 aside (which in 1991 sold approximately 7.1 million copies itself under very unique circumstances) you are assuming some spectacular market growth for Sleepwalker to be selling 500,000 copies per issue, let alone 1 million copies per issue.  
I made a very quick search and haven't yet turned up individual sales figures per issue for 1991. However, in the Marvel Comic Group IPO published June 1991 reports monthly sales for Marvel Comics in the later half of 1990 were 8.7 million. The same documents reports in the first half of 1990 sales were 6.6 million per month. That gives us an annual average of 7.65 per month for 1990, modest growth over the 1987 tally. Such claims for Sleepwalker are unsupportable.