I think that basing the gift on the cost of the meal you are receiving is silly! So the richer people get more gifts? The purpose of inviting you to to have you share in the special day, not to elicit a gift! If you suspect you are being invited to anything for any reason other than the joy of your presence, they don't go!
You can send a gift afterwards - it's absolutely acceptable for up to a year, although I don't like to push it out that far. So for the friend, if you will have hotel fees, give a gift afterwards, when you've accumulated a few more paychecks. Even though she received everything on her registry, she will definitely need and want stuff afterwards. I would get something you think she will like at a big chain store (department store, Bed Bath & Beyond, etc.), knowing that she can easily exchange it if needed.
For the boss's son, I would give a gift sooner. Be careful giving money or cards at the wedding - friends of mine had their basket of envelopes cleaned out (either by a guest, the person "guarding" it, or a staff member at the club), and the poor couple lost over $1000 from well-meaning friends. Gift cards and cash are completely negotiable by anyone. If you don't want to put a price tag on something for your employer, then buy a gift, again at a large store that will have many options for return/exchange for the couple, whose taste you do not know.
Give what you can afford. $50 for your friend maybe, and $30-$50 for the boss's son.