Learn German: Das Gebot ist leider zu hoch. - The bid is unfortunately too high.

While both words relate to offering something, das Angebot is the general word for an "offer" (like a job offer or a sale in a supermarket). Das Gebot is more specific: it usually means a "bid" at an auction, or a strict "commandment" (like the Ten Commandments).

Since this lesson is about unfulfilled wishes, how would I follow this sentence up with "If only it were lower!"?

You would use the Konjunktiv II (Subjunctive II) of sein along with a particle like nur or doch. You can start with "if" (Wenn es doch nur niedriger wäre!) or drop the "if" and put the verb in the first position (Wäre es doch nur niedriger!).

I often see the word for "high" spelled without a "c", like hohes. Why is it hoch here?

Hoch is an irregular adjective. It keeps its "c" only when it has no grammatical endings. Because it comes after the verb ist here instead of directly before a noun, it takes no ending. If you placed it before the noun, it would get an ending and lose the "c", as in ein hohes Gebot (a high bid).

Could I put the adverb at the very end and say Das Gebot ist zu hoch leider?