Depends where you are. $1 in the States per case. I do a rough link to the pound according to local currency so would do A$2 in Oz, but only about the equivalent to 50p in countries where 50p is an awful lot of money such as India or parts of Africa.
We always carry extra euros/pounds for tipping at the airport for porters who carry luggage etc. My husband gets assistance at the airport either a wheelchair or buggy and we always give a tip,the service is excellent so it is worth giving especially as some of these porters don't be paid very well. If a porter carries your luggage to the room as they do on cruises they deserve a tip, five euros etc is
reasonable, Varies from place to place and depending on how you appreciate the service.
To be honest, I can't see any reason to tip anyone, especially in exorbitantly priced restaurants and hotels. If tipping stopped like in Singapore and NZ, the porters etc. would have to be paid a decent wage or nobody would do it.
I agree with wildwood to a certain extent. I also find it a bit tacky to hand money to someone directly, especially when its such a small amount like £1 or £2, but then its not appropriate to give any more, so I tend to leave it.