A birthday does not have to cost a month's rent to feel like an occasion. What people remember is being thought about — and attention is free.
Spend on the moment, not the props
Decorations photograph well and are forgotten by the weekend. A shared meal, an afternoon someone actually took off work for, or a room booked long enough that nobody is watching the clock will outlast any balloon arch.
Let people contribute what they have
Not everyone can chip in the same amount, and asking them to is how a celebration turns into an obligation. Give people a way to give time, a recipe, a playlist or a lift instead of cash, and more of them will say yes.