Notes on the synthesis and use of oligonucleotides:
Sigma Alrich - oligonucleotide learning center: http://www.sigmaaldrich.com/life-science/custom-oligos/custom-dna/learning-center.html
Purification, quality control and error rates
Advances in the Synthesis of Oligonucleotides by the Phosphoramidite Approach: http://www.sciencedirect.com/science/article/pii/S0040402001887524
According to this page, Phosphoramidite oligo synthesis currently has an error rate of 1 in ~160. This corresponds to a success rate of 0.99375 per step. For a 50-mer synthesis, this would result in ~73 % of products having the correct sequence.
Assuming a success rate of 0.99, a 50-mer would give 61 % correct products.
According to this page, "Because truncations are common, and deletions do occur to a much smaller degree, we strongly recommend PAGE purification for any oligo used for cloning, mutagenesis, or other sensitive procedures regardless of oligo length." and "Base substitutions are extremely rare."
Conclusions in brief:
Oligomer synthesis services
Sigma Aldrich price list: http://www.sigmaaldrich.com/canada-english/sigma-genosys-canada/custom-dna-synthesis.html
LeProust E. M., et al. (2010) Synthesis of high-quality libraries of long (150mer) oligonucleotides by a novel depurination controlled process. Nucleic Acids Res. http://nar.oxfordjournals.org/content/38/8/2522.full
Enzymatic production of 'monoclonal stoichiometric' single-stranded DNA oligonucleotides: http://www.nature.com/nmeth/journal/vaop/ncurrent/full/nmeth.2503.html