I don't practice in probate court but my advice is to answer the statement honestly. If the statement is requesting all income and expenses, I would think you would need to state either that you pay $650 a month in rent or pay $1,100 but then receive $650 in additional income. Either way, you are only out $650 a month for rent, not $1,100.
You need to answer the financial statement truthfully, and honestly. It will be signed under the pains and penalties of perjury, and there can be consequences for trying to mislead the court as to what your financial status is. While you do not state the specific form you are completing, the Short Form has a line for other, and household member's contributions. If your GF contributes to the expenses, it will need to be shown.
DISCLAIMER: This answer is provided in response to a "hypothetical" question and provided for general, informational purposes and does not create an attorney/client relationship. The information presented is not legal advice and may change based additional information and research. It is recommended that you speak to an attorney to discuss your specific legal issues.