
billig
cheap
Inexpensive or associated with low costs; not expensive.
Er hat ein sehr billiges Flugticket im Internet gefunden.
He found a very cheap plane ticket online.
cheap
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cheap
Inexpensive or associated with low costs; not expensive.
Er hat ein sehr billiges Flugticket im Internet gefunden.
He found a very cheap plane ticket online.

cheap
Of poor quality, inferior, or crudely made. This meaning has a strongly negative connotation.
Die Reparatur war eine billige Notlösung und hielt nicht lange.
The repair was a cheap stopgap solution and didn't last long.

cheap
Mindless, lowbrow, or tasteless. Refers to behavior, jokes, or excuses.
Das war eine billige Ausrede, die ihm niemand geglaubt hat.
That was a cheap excuse that no one believed.

fair / proper
An outdated expression meaning just, lawful, or appropriate. Today, this meaning is found almost only in legal or historical contexts.
Es ist nur recht und billig, dass er für den Schaden aufkommt.
It is only fair and proper that he pays for the damage.
Careful: 'billig' (cheap) can sound negative. Although it can mean 'preiswert' (inexpensive), it often carries the secondary meaning 'of poor quality' or 'lowbrow'. The context determines which meaning is intended.
Er kauft nur billige Kleidung, die schnell kaputtgeht.
He only buys cheap clothes that fall apart quickly.
When 'billig' (cheap) is used neutrally for 'preiswert' (inexpensive), it often appears in contexts where price is the main criterion. Typical combinations are 'billige Flüge' (cheap flights), 'billig tanken' (to fill up cheaply), or 'billig übernachten' (to stay overnight cheaply).
Im Internet kann man oft billiger reisen.
You can often travel more cheaply online.
In the negative meaning 'niveaulos' (lowbrow/tasteless), 'billig' (cheap) is often combined with abstract nouns that describe behavior or communication. Common examples are 'eine billige Ausrede' (a cheap excuse), 'ein billiger Trick' (a cheap trick), or 'ein billiger Witz' (a cheap joke).
Das war ein billiger Trick, um nicht bezahlen zu müssen.
That was a cheap trick to avoid having to pay.
The old meaning 'gerecht' (fair/just) or 'angemessen' (appropriate) is now only common in the fixed expression 'recht und billig' (fair and proper). It means that something is fair and appropriate.
Es ist nur recht und billig, dass du dich bei ihr entschuldigst.
It is only fair and proper that you apologize to her.
cheap / favorable
Günstig and billig can both refer to a low price. Günstig usually sounds neutral or positive, while billig can additionally suggest poor quality. For a low price without a negative judgment, günstig is the right choice; for noticeably inferior goods, billig can express this criticism.

Das T-Shirt ist billig und wirkt wegen der dünnen Nähte minderwertig.
The T-shirt is cheap and looks inferior because of its thin seams.
Das T-Shirt ist günstig und trotzdem gut verarbeitet.
The T-shirt is inexpensive and still well made.