How can I tell

My colleagues are planning to have the farewell party on tomorrow
Which I don't wish to join
First, because the location for the party is far from my house
It's not convenience for me to go home after the party due to I don't have my own transport
Second, I don't really wish to join them not because of I don't like my colleague
It's because of I don't have much money to join the party
I just came back from my hometown and I used a lots of money o the trip
Their salary is more higher than mine
Maybe double up...

I feel very difficult to reject them 
I don't know how can I tell them
Sigh...