Translation:This lady only has expensive clothes.
Ah ok, I get your question now. They are different. Heel veel is an amplifier, this is more than veel. Alleen maar is like erg in heel erg veel it's an extra filler word that is optional and doesn't really change the meaning, but it makes the sentence 'nicer' and better flowing. Another example where there is a bigger difference between using this word is: Zeg het maar vs Zeg het. The former means How can I help you or Just tell me, the latter means Say it.
The meaning of "maar" is more like "but", so you can't use it on it's own to mean "only". A meaning like "we hebben maar (something)" would mean something like "We only have (something) and nothing else at all".
"Ik heb maar één sok!" = "I have only one sock!", or in a more old-fashioned way "I have but one sock!"
So "deze dame heeft maar dure kleren" would imply that it is a problem that the lady only has expensive clothers, or that the only thing she has at all is expensive clothes.