Note: the only difference from the point of view of passing the exercise is that it wants to check whether you've learned the verb "to cost" in Ukrainian. So if you skip it, it won't be accepted.
In real life, these two sentences mean the same and are used interchangeably.
Це can means "this" as a "noun pronoun" (I don't think it's a real linguistic term, but let's run with it). As in, grammatically it works as if it was a noun. E.g. This is a cat = Це кіт.
Це can also mean "this" as an "adjective pronoun". It's a neuter "brother" of the masculine цей and the feminine ця. E.g. Це молоко = This milk.
So, since they overlap for the neutral gender, Це молоко = This milk = This is milk
Which brings us to цей. Means "this" as an "adjective" for masculine nouns. This cat = Цей кіт ≠ This is a cat.
Hope it helps, if it doesn't, ask again :D
What examples can be given for the different use between "Це дешево" and "Це коштує дешево". I again come back to the lack of context of the phase. I originally translated the problem as "This does not cost very much." Then thinking that this was too complicated a translation, I looked at the hint and changed the response. So, if, as Vinnfred pointed out, the literal translation is not appropriate then any translation which, within reason, approaches the intent of the phase should be acceptable. Finally, translating "it is cheap" back into UA can take several forms (нічого не вартий, підлий, не заслуговує поваги, as well as, коштує дешево) when the context is not known. Yes, I am a little frustrated. :) I'll live with it. LOL.