Catering for office worth more than 4 months of additional work

I'm an admin who recently has taken on the majority of work for another department. They said that it would be until the end of financial year and that I would be renumerated for the increase in capacity. 6 weeks later, after I had to chase I get a letter stating my “increase” which works out at around an additional $200 a month. Four months of work for essentially $800. Later that day I get asked to cater for an upcoming function for our office. The total was $900 just on food. Cannot believe I am worth less than a few platters of cold meats.