At my work (office of just under 30) we put �1 in for birthdays and usually �5 for weddings, leaving and babies. Personally, I would just say if you can't afford to match the others as there's nothing worse for the collector discovering someone has filled your envelope with shrapnel - one guy at our place always ends up putting 62p in or something, which is just rude.
Until last year at work, it was always up to the admin staff to organise collections, but after one year of having to organise and decide what to buy for 5 birthdays all in July, I had a hissy fit and now I just email staff with whoever has a birthday coming up and let someone else organise it!