"Kolik stojí ta motorka, o niž se Matěj zajímá?"

Translation: How much does the motorcycle Matěj is interested in cost?

[dsarkarati]

"How much does the motorcycle in which Matej is interested cost?" is a grammatically correct translation.

[walkinwolk]

In English, it is not necessary to say "cost". The sentence can be phrased as; "How much is the motorcycle that Frantisek is interested in?"

[VladaFu]

And it is, surprise, accepted.

Note that in Czech you can also say "Za kolik je ta motorka...

[chartsman]

How to say "how much is the motorbike that interests Matej?" then?

[BoneheadBass]

FWIW, since I am learning, too... If I saw your sentence as a translation exercise, I'd probably go with "Kolik stojí (ta) motorka, která zajímá Matěje?" (Might be good, might not!)

[VladaFu]

Correct. Also "Za kolik je ta motorka...".

We might add this wording too as we often mix zajímat and zajímat se and to be interested and to interest in earlier skills.

edit: added

[chartsman]

Thanks, it would be good. Both constructions exist in Polish as well but looking for a difference in meaning is like splitting hairs but I get it now.