Here are reported sales of the keyword, as far back as 2010:
Domain TLD SalePrice SaleDate SaleThru Wrds Len Num Hyp Link Source
Gas mobi $9,100 Dec 07 Sedo 1 3 N N
http://Gas.mobi Sedo
EuropeanGas com $2,500 Jun 12 AfternicDLS 2 11 N N
http://EuropeanGas.com DNJournal
EGas com $3,901 Jun 12 NameJet 2 4 N N
http://EGas.com DNJournal
FloridaGas com $6,400 Jun 12 AfternicDLS 2 10 N N
http://FloridaGas.com DNJournal
OilAndGasInvestor net $1,000 Apr 12 AfternicDLS 4 17 N N
http://OilAndGasInvestor.net DNJournal
BuyGas com $3,500 Mar 12 Sedo 2 6 N N
http://BuyGas.com Sedo
CarGas net $1,000 Aug 11 AfternicDLS 2 6 N N
http://CarGas.net DNJournal
GasGrills org $4,410 Apr 11 Sedo 2 9 N N
http://GasGrills.org DNJournal
GasStation com $13,600 Feb 11 Sedo 2 10 N N
http://GasStation.com Sedo
GasPreisVergleich org $1,529 Jan 11 Sedo 3 17 N N
http://GasPreisVergleich.org Sedo
CarGasMileageComparisons com $850 Jan 11 Sedo 4 24 N N
http://CarGasMileageComparisons.com Sedo
GasSprings co.uk $1,367 Jan 11 Sedo 2 10 N N
http://GasSprings.co.uk Sedo
GasSprings eu $2,250 Dec 10 Sedo 2 10 N N
http://GasSprings.eu Sedo
IncreaseGasMileageProducts com $1,500 Dec 10 Sedo 4 26 N N
http://IncreaseGasMileageProducts.com Sedo
NaturalGas co $2,550 Oct 10 .CO Landrush 2 10 N N
http://NaturalGas.co DNJournal
SupaGas com $6,000 Sep 10 Sedo 2 7 N N
http://SupaGas.com DNJournal
SwedishBioGas com $4,464 May 10 Sedo 3 13 N N
http://SwedishBioGas.com DNJournal
GoldGas com $3,688 May 10 Sedo 2 7 N N
http://GoldGas.com DNJournal
GasPreisvergleich org $1,541 Apr 10 Sedo 2 17 N N
http://GasPreisvergleich.org DNJournal
GasGrill org $1,311 Feb 10 Sedo 2 8 N N
http://GasGrill.org DNJournal
And 'gas; is bigger a keyword than 'oil.' I could see $5,000 as reasonable here.