Budget time is all about what where your coin is going to go.
Sometimes it may be sourced from existing funds, other times it may be allocated from projected cashflows. Either way, appropriate budgeting will at least minimise the impact of those planned and unplanned events.
Marketing is another budget item and it needs to compete with other items for a finite resource. When formulating your marketing budget consider the following:
What has previously worked and what has not.
What has generated new customers.
What has improved existing customer values?
Cross promotion results.
Stock clearance/liquidation success.
These are all measurable items where the results are largely tangible against a reasonably prompt turn-around.
There are not too many variables to setup to begin with, the relative success or not of the campaign is not influenced by a large number of variables, although, those variables that do exist should be identified and accounted for if they came into play during the campaign. This is especially true for campaigns that work and those that failed – the latter being important to identify and not repeat, the former something that needs to be replicatable in maybe different environments.
Budgeting also works well when looked at as a business review process. Budget comparisons between this and the last year can provide some interesting insights on which areas may need additional consideration and those areas that may have had generous budgets.
The process of budgeting should not be seen as a beginning of financial year process. Ideally it might be better to view and adjust the budget continuously, however this may generate unsustainable overheads. My recommendation, half yearly reviews with quarterly report generation should provide the sort of data you need to make informed decisions.