You can get per-month sales info from the Sales Activity report as a premium developer (free), split by currency type (USD/CAD/GBP). This includes data for free channels if you select that, and is also exportable as a CSV.
You could add the prior months totals (just save them as a column in a spreadsheet) and add the current month's values for a quick count. Unfortunately that will be total sales, not current installs (which fluctuates as people uninstall).
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