Re: the bolded. The reason I think the writer is rude is *not* because he ordered an expensive meal. It's because he is complaining about other people ordering an expensive meal that he will have to help pay for (unless of course he does the mature, responsible thing and actually deal with it - by asking for a separate check or addressing the overpayment issue), while he is doing the exact same thing But he's not just doing it because he happens to want the expensive meal. And honestly, his "strategy" is more likely to exacerbate the problem for himself in the future. Because if everyone thinks like him, then everyone will keep ordering more and more expensive things each dinner out. And the only one who wins will be the restaurant.
The author explicitly states that he used to order on the lower end of the scale but, being unable to change the attitude of the group -- the majority of which were ordering on the high end -- he decided to stop subsidizing their meals while he goes without on his own.
"I developed this system after too many birthday dinners where I went home poor and hungry. This way, at least, you get the food you want."
Honestly, I can't blame him. Would you want to order $30 worth of food only to be expected to contribute $170 to the meal? That's nearly SIX times the amount you ordered. Meanwhile, you get only a small dish or maybe just an appetizer and drink water, so you're not completely full (unless you're a very small eater to begin with), don't get the drink you wanted, and have to eat Ramen for a *week* at the end of the month because the extra $140 has to come from somewhere in you're budget and if you're on a tight one (as many grad students are -- I seriously don't know a single one, who doesn't have a trust fund or very generous parents, who hasn't overdrawn on their bank account at least once to cover basic necessities) it probably means skipping on a bill or skimping on groceries.
Can you decline the invite? Yes, to the risk of you're friend feeling very hurt and like you're not as close as s/he thought you were.
Can you insist on only paying your portion? Yes, but you come across as a cheapskate and jerk because you're begrudging others. This can also impact your relationship
with your birthday having friend. (And, honestly, I don't think anyone can argue that people aren't looked down upon if everyone else agrees to split the check evenly and they insist on splitting their portion only. People who go against the grain with money in such a way they're trying to pay less aren't looked upon fondly. It's more "I'm splitting the check evenly and I only ordered $150 dollars worth of food, so what's wrong with you that you won't suck it up for the group?")
At which point the only halfway reasonable option is to order food enough that you'll not be paying six times your own bill.