ATP cost for gene expression
The cost of transcribing and translating a hypothetical average gene in yeast has been calculated as 551 activated phosphate bonds ~P per second (Wagner, 2005). 


  The median length of a yeast RNA molecule is 1,474 nucleotides, and
  the median cost of precursor synthesis per nucleotide (derived from
  the base composition of yeast-coding regions) is 49.3 ∼P. With a
  median mRNA abundance of R = 1.2 mRNA molecules per cell and a median
  mRNA decay constant of dR = 5.6 × 10−4 s−1, the mRNA synthesis costs
  calculates as 49.3 × 1,474 × 1.2 × (5.6 × 10−4) = 48.8 ∼P per second
  and cell. This is a fraction 48.8/1.34 × 107 = 3.6 × 10−6 of the total
  RNA synthesis cost per second. The median length of a yeast protein is
  385 amino acids, with a combined biosynthesis and polymerization cost
  of 30.3 ∼P per amino acid. The median abundance is 2,460 protein
  molecules per cell. No currently available data allows a meaningful
  estimate of the median protein half-life, but a protein of an
  intermediate half-life (see below) of 10 h (decay constant dP = 1.92 ×
  10−5 s−1) yields an overall synthesis cost of 30.3 × 385 × 2,460 ×
  (1.92 × 10−5) = 551 ∼P s−1.


For your question about a single gene, the cost would be 49.3 * 1474 ~P for the mRNA and 30.3 * 385 ~P for the translation, which would result in around 84 thousand ~P. This is probably a very misleading statistic as you can transcribe multiple proteins from a single mRNA. 

How the cost of mRNA synthesis and translation are calculated is described in detail in the paper. A large part of the cost comes from the synthesis of the basic building blocks, the nucleotides and the amino acids.




Wagner, A. Energy Constraints on the Evolution of Gene Expression. Mol Biol Evol 22, 1365-1374 (2005).