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.

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.
bad
'schlecht' and 'billig' can both say something negative about quality. 'schlecht' is more general and means that something does not work well, has many flaws, or does not meet expectations. 'billig' can mean the neutral 'not expensive', but in its negative meaning it means: something seems inferior, crude, or poorly made. If you only mean the quality, 'schlecht' always fits. 'billig' fits especially when something looks or feels as if it was made very simply and without care.

Die Tasche sieht billig aus, weil das Material sehr dünn ist.
The bag looks cheap because the material is very thin.
Das Hotel war teuer, aber der Service war wirklich schlecht.
The hotel was expensive, but the service was really bad.