Approximation, Rough Estimate
noun, verbal noun, の adjective
An approximation of the calculation is an approximation or rough estimate.
がいさん
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
旅行費用の概算を教えてもらえますか。
Could you give me a rough estimate for the travel costs?