"This is a bad price."
Translation:Este es un mal precio.
".....un precio malO" would be grammatically correct (because "malo" drops its "o" only before a masc. sing. noun). You could try that next time if you can access that sentence again. I don't know, but there may be something about the meaning of this sentence that disallows your suggested word order.
Mal is something called an "apócope" which is when a sound is dropped. In this case "malo" without the o at the end. Also "mal" is being used here as an adjective rather than an adverb. Usually in Spanish the adjective comes after the noun, but if you put "mal" at the end of a sentence it USUALLY changes from adjective to adverb.
Ex (adverb): Este es un precio mal puesto. (This is a price wrongly put) Yo conduzco mal. (I drive badly.) Lo estás haciendo mal. (You're doing it wrong.)
Ex (adjective): La mala suerte. (The bad luck.) El mal hablar no es bueno. (Wrong-talking is not good.)