Questions & Answers about zhè shì gōngběnfèi.
Because 这 is being used by itself as a pronoun meaning this.
Compare:
- 这是工本费。 = This is a processing/basic-cost fee.
- 这个工本费... = this processing fee...
When 这 directly modifies a noun, Mandarin usually needs a measure word/classifier, such as 这个, 这本, 这张, and so on. But here 这 stands alone, so 这 by itself is correct.