That doesn't make any sense at all.
In a normal lifestyle, 5000 nuyen are, according to Jack, divided in
4000 for fix costs (rent, matrix access, insurances, ...)
1000 for variable costs (i.e. food, transportation, ...)
If we share a lifestyle, 500 of our variable costs are to be borne by each and every one (i.e. food), whereas some of the variable costs can be divided (gas, electricity, ...).
This is why we pay an extra 500 nuyen for each additional individuel. Thus it is (5000+1500)/4 = 1625.
According to Jack, 1000 of our costs both personal (food, fuel) and divided (gas, electricity) will not have to be paid because we don't live here. Thus, our monthly costs are reduced to 625, which is basically only rent and some unavoidable costs.
Or did I miss something?