I can try and answer some of these questions. You could try uploading the image to some other site and providing a link, but I think I know what it is from a quick google image search.

I suspect what you have is essentially a souvenir item from a pilgrimage. Many holy sites produced and sold commemorative jewellery for pilgrims. Usually they sold little badges, but rings were not uncommon. Some examples of such items include this pin badge from the shrine of John the Baptist at Amiens Cathedral, this little flask from the shrine of Saint Sergios in Syria which was supposed to hold holy water, and this rather fetching gold ring, which was sold at the final stop on the famous Camino de Santiago pilgrimage route.

The design of the Bethlehem star seems to have been a rather generic symbol for 'hey, I'm a pilgrim' rather than anything specific. The widespread use of your ring's rather generic design means that it is highly unlikely that anyone will be able to work out exactly where it came from, but your friend's claim that it came from Acre is entirely plausible.

Now, onto your questions.

"These rings don’t seem to be so rare, nor very expensive, so I’m wondering how common these rings were back in the day"

They were everywhere, and they were cheap. Many popular sites sold tens of thousands of them a year. We do not know how much your specific ring would have cost, and different vendors would have no doubt charged different prices, but we do have some records from other pilgrimage sites. At the shrine of Thomas Becket, the most popular pilgrimage site in England, you could buy a dozen pewter badges for a penny. In terms of modern price, we are talking about maybe 20-50p for a badge - pocket change, basically.

"Is it true that only high placed people would wear this ring, or were these rings handed out to whomever signed up for the crusade?"

As is implied by the price, these rings could be bought by pretty much anyone. Yours is made of bronze, so it was one of the cheap ones. It's worth noting that the owner may have had absolutely nothing to do with the crusades, they might have purely been a pilgrim. It's very common for auction houses to list anything from the medieval Holy Land as 'crusader' even if it has no association with a crusade. If your friend says it's from the 11th century, then that raises some questions. The Holy Land was not particularly safe in the late 11th century, as conflict between the Fatimid and Abbasid caliphates saw the Holy Land become a war zone, and both Islamic and Christian sources claim that pilgrims were harassed by bandits and soldiers. If it was the 11th century, it probably predates the First Crusade. It could also have belonged to someone on the First Crusade, which would be rather exciting, but there's no way to tell.

"What exactly did such a ring signify? Was it somewhat prestigious, or was it the medieval equivalent of “I went to Acre and all I got was this lousy ring""

Well it's not a lousy ring, but it wasn't prestigious either. The ring signified that the owner was a pilgrim, but that's about it. There were certain designs specific to certain sites, saints, and routes, but the Bethlehem star was not one of them.

"Would this ring, and others like it, have looked the way it looks now? Or would they have been coloured or have been inlaid with gold leaf or something?"

Probably not, as that would have increased the cost as well as the labour involved in making them. Silver and gold decoration could be done, but it does not appear very often; remember that many sites had to make tens of thousands of these a year. Often these pieces of jewellery were made by simply pouring molten metal into a mould, as that was the only way to produce enough of them to meet demand. It was far more common to simply sell bronze, silver, and gold versions of the same items - sometimes from the same moulds - because that way they could meet demand.