Even though there's an approximate market price, since there are no precise standards, if there is demand for an object the price keeps going up and up, while if there's no demand it can't even be sold cheaply.
確かにそうですよね。値段がスキル、時間、素材、名声などに基づいているならまだ理解出来ますが、現代美術の場合には値段がほぼ恣意的に見える場合も少なくないです。
「美術品の値段はあってないようなものだ」 ＝
"The price of art seems to lack any solid basis."
美術品の値段 ＝ the price of art; the price of works of art
lack = 欠く
seems to lack any solid basis ＝ 明確な根拠を欠けているようです
「一応、値段の相場はあるけれど、明確な基準がないので、欲しい人がいればどんどん高くなるし、そうでなければ安くても売れないという場合です」 ＝
"Even though there's an approximate market price, since there are no precise standards, if there is demand for an object the price keeps going up and up, while if there's no demand it can't even be sold cheaply."
値段の相場 = market price
一応 = approximate; tentative
明確 ＝ precise; clear
明確な基準がない（ので） = (since) there are no precise standards
需要があれば ＝ if there is demand
どんどん高くなる = the price keeps going up and up; the price keeps rising
アメリカ合衆国 
