Do I have to buy a lavish gift?

My H agreed to be in a friend's wedding about four months ago. When he agreed, the bride and groom told us that the attire for the groomsmen would be low-key sort of beachy attire (khaki shorts and tux shirts). Now, two weeks before the wedding the bride went all bridezilla and decided that the men all had to be in tuxes, and now we have to spend about twice what we had budgeted for and only have two weeks to come up with the extra $$. We are on a VERY tight budget, and my question is this - since they sort of sprung this on us, do we have to spend as much on their wedding gift as we normally would on someone else? Basically the only place the extra $$ for the tux can come from is from what we budgeted for their gift. Is this tacky to not buy them something expensive, or am I right in being a bit miffed because of the sudden change in everything and the upheaval it has done to our whole budget??