Daemons units tend to be a trifle cheaper than most 40k units, since they have to share their line with the cheaper Fantasy line. Of course, they're also a horde army (unless you go with a purely FMCspam route, or something).
Do you carry over unspent change between months? (strikes me like you would, probably, but...)
If you want purely a "most points per mini bought" army you might want to go an elite army like Grey Knights, Plague Marine CSM, Dark Angels or the aforementioned Daemons FMCspam...
Of course, really, when starting a new army, I can't help but think that the most important thing is to be passionate about the army (background, minis, what have you)--what interests you to be able to play is more important than squeezing the most bang out of your buck. Tell us what armies you're interested about in the first place, perhaps: we could give advising on what units you ought to buy within that army to get the most effective of a little force...
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!