I hope this helps you out :).
From what I have seen, goeie only seems to replace the inflected adjective goede, but never the uninflected adjective goed. Given that it ends with what seems to be the inflection e, this makes perfect sense. Since the adjective lost its inflection when the new noun goedkoop was formed ("een goede koop is goedkoop"), I guess goiekoop would be just wrong.
The real problem is not that it is confusing but that you must spend a lot of effort on learning the (essentially arbitrary) genders of all Dutch nouns. It's not intuitive for native English speakers because English has given up the distinction long ago; it only survives in he vs. she vs. it and other pronouns. But English too did go through a stage in which you would have said something like "The (= de) man and the (= de) woman and that (= het) child" because man was a masculine noun, woman was a feminine noun, and child was a neuter noun.
Fortunately, the grammatical distinction between masculine and feminine has disappeared in standard Dutch. It only survives in Flemish (variant of Dutch spoken in Belgium), so you can just ignore it.
But ideally you should still learn which nouns 'have gender' (i.e. are masculine or feminine), and which 'do not have gender' (i.e. are neuter). Nouns with gender, like soep (which I guess is feminine in Flemish because it is in German) take de, and nouns without gender, like vlees, take het.
So much for the bad news. Now we come to the good news. It's not as bad as it could be.
Since masculine, feminine and neuter were once roughly equally distributed (they still are in Flemish and in German), two thirds of all nouns have gender. Therefore a good strategy is to just remember the most important nouns without gender and treat the others as having gender. You will be correct most of the time and nobody will be confused. Even Germans have trouble with Dutch noun gender because it's not always the same, and it's even harder for native speakers of other languages such as French, which also have noun gender but one that very often doesn't agree with Dutch noun gender. (This is in part because Latin often had different noun genders than Proto-Germanic, and in part because in French it was masculine and neuter that merged, not masculine and feminine.)
We speakers of gendered language are totally used to people getting the genders wrong. This will not cause confusion, and we will barely notice it. Some people will correct you, but if you don't like that, just tell them to stop. Of course Duolingo and teachers have different standards. They try to make you get this completely right. Just humour them to some extent, but do not take them too seriously.
PS: The standard trick for learning this properly is to always learn nouns along with the definite article. Instead of learning vlees and soep, you learn het vlees and de soep. It turns out that the mental effort for this is practically the same except for those words such as soep which are already very familiar from English.
Inexpensive sounds pretty formal to me, whereas goedkoop is a very basic word. But to judge from the Dutch-German dictionary I consulted, goedkoop can also be used in this formal sense. So yes, your sentence seems to be a valid alternative. Although it's not surprising if it is not accepted yet, as for most people it's not the first translation that comes to mind.