A proof becomes a proof that it is produced according to the specifications of the latest revision of the proofing standard ISO 12467-7 and that it is within the tolerances of this standard. The current revision is ISO 12647-7:2016, which has been tightened even further with this standard and has been supplemented by a certified edition of spot colours such as PANTONE and HKS.
But what makes a certified proof cheap? Well, the low price. Proofs are printed on certified proof papers on very high-quality pigment inkjet printers using mostly expensive proof software and measured with spectrophotometers. So how can you produce cheaply here?