A Squib is someone who was born into a wizarding family but hasn’t got any magic powers. Kind of the opposite of Muggle-born wizards, but Squibs are quite unusual. - Chamber of Secrets
They are already extremely rare, but they will happen even in pure blood families, so I think that precludes the idea that Muggle-born wizards are more likely than not.
Some pure-blood families, such as the Blacks, disown any Squib members and remove them from their family trees, as they removed Marius Black. - Harry Potter Wiki (referencing Black family tree).
That doesn't stop some families for blaming Muggle-born wizards for a squib being born.
Some magical families would often blame any Muggle parents for a Squib's birth, as Orford Umbridge and his witch daughter would blame his Muggle wife, Ellen Cracknell, for being the fault of the son being born a Squib. - Harry Potter Wiki
Additionally the Weasley's have a second cousin who is a squib which they never talk about.
Squib's might have a better chance of producing magic born children, as in an early draft of Goblet of Fire, the second cousin had married a muggle and had a witch daughter. Perhaps that is the source of Muggle-born wizards, they are descendants of squibs who were cast out of the Magical World and their genes eventually came back into play.