Learn Russian: Картошка стоит недорого.

In Russian, картошка is a collective noun, meaning it is used in the singular to refer to potatoes in general. Because the noun is singular, the verb стоит must also be in the singular form.

The verb стоит can mean 'stands' or 'costs'. How do we tell them apart?

By the spoken stress and the context! If the stress is on the first syllable (сто́ит), it means 'costs', which is the only meaning that makes sense with a price or value. If the stress is on the last syllable (стои́т), it means 'stands'.

Why does this sentence use the adverb недорого instead of the adjective недорогая to match the feminine noun?

In English, we often use the verb 'to be' with an adjective ('are inexpensive'). But in Russian, the verb here is стоить (to cost). Because you are describing the action of the verb (how much it costs), you must use the adverb form ending in -о rather than an adjective.

Can I write недорого as two separate words (не дорого)?

In this specific sentence, it is much more natural to write it as one word. Written together, недорого is a direct synonym for 'cheaply'. Writing it as two words (не дорого) is usually reserved for when you are making a direct contrast, like saying 'It is not expensive, but rather cheap' (не дорого, а дешево).