There can possibly be a simple approach here. In many cases, it is well known if the family making the simcha is struggling or obviously wealthy, and we can act accordingly. (NOT THAT WE EVER REALLY KNOW HOW MUCH MONEY SOMEONE HAS IN THE BANK FOR REAL—sometimes those that appear wealthy are actually in debt, nevertheless,)For example, if the family does not spare any expense to make the wedding as lavish as possible, eg,fancy invitations, live flowers, eight-piece band, custom made gowns for everyone and two photography crews, etc), then they can consider bringing boys/girls in to be part of the wedding expenses. (often, these weddings are held in a luxury hotel with not-so-convenient access). For these families, the extra expense this will entail is so minor, as to almost be inconsequetial. To these weddings, for the ba’al simcha to arrange transportation would be mentchlich and generous, being that these expenses do add up for the bochurim.
On the other hand, for those families that are obviously struggling (and its no secret when they are!) the young friends should have the sensitivity not to ask for transportation accomadations. They need to remember that it is the parents paying for the wedding, and as much as the chosson/kallah would love to bring the friends in, it may not be up to them. a sense of entitlement would be entirely innapropriate here. At the same time, they should make the effort to come whenever possible to wholeheartedly share in the simcha, despite it costing them money.
Then there’s the case where the bochur or girl guest comes from a well-to-do family, and does not NEED to accept the money offered by the family. Someone, who comes from a very well to do family once told me that he flew in several friends from yeshiva for his wedding. However, what he resented was when a certain bachur who could very well have afforded to come on his own, asked to be reimbursed. The chassan was too aidel to argue, but he felt the friend was just taking advantage of his generosity, and that it was not mentchlich.
On the other hand, when a family is not offering to bring in guests, and the boys arrange their own rides, it is sometimes in place for the chassan/family to offer reimbursement to a bachur (or more–depending on their means and the need) who really cannot afford it but should really be at the wedding, being that he is a close friend or whatever. his may be the families choice, but should not signal to the others that they should expect the same.
My point here (in case its not obvious) is that there does not need to be a one-size-fits -all solution. What works for some families, may not work for all. People can try to use their judgement and sensitivity to assess the situation properly. Once the different aspects and problems are known, each bochur and family can apply their own common sense.
As an aside, to all you bochurim: If the chassan does not offer, DO NOT ASK or expect to be reimbursed. Just do the best you can. It can be extremely uncomfortable for the chassan when he knows his parents will not pay for it, but his friends are expecting it.