Quoted:
Can anyone tell me what company's manufacture Mil-Spec ARs
NOBODY
There is no Mil-Spec for an AR-15.
If you think 'Mil-Spec' means will interchange with 'standard' parts well just about ALL ARs currently produced will do that (with some exceptions of some parts on older Colts)
Now some AR makers follow Colt's M4 TDP (Technical Data Package) better than others. They usually follow the Mil-specs for many of the materials and finishes (but NOBODY makes a complete 'Mil-Spec' rifle).
Brands like Colt, BCM, DD, LMT are known to follow the TDP fairly well (not perfectly, but fairly well).
It must be Mil-Spec and as low in price as possible.
The closer you get to the TDP and follow the 'Specs the more it will cost. Period. However, you can get a reasonable facsimile of a military M4 from many of the vendors with the deviations in QC, materials, and finshes not observable to the average AR enthusiast, let alone the average person.
Some 'good' M4 carbines that are more reasonably priced include
Spikes
Stag
Palmetto
S&W
I'd also point out in the past I've had very good luck with Bushmaster (indeed during the AWB they were the only place you could get a decent chrome lined M4 type barrel). Not sure these days with the factory being moved and their consolodation with the rest of Cerbus.