The summary field can still be used if you define a relationship that matches to all the budgets table records that you want to summarize.
Purchases::anyfield X AllBudget::anyfield
will work for that purpose if AllBudget is a new occurrence of your Budget table. The cartesian join operator matches any record in Purchases to all records in AllBudget so this gives you a match to all records in the budget table. That way, putting the summary field, taken from AllBudget, on your layout will show a grand total of all records in the Budget table.