"Buenos dias" from California!
I think the "proposal" you're referring to "quotation/estimate" in English (US) for photography assignments. In English, when a photographer is asked to bid on a project, the photographer is required to submit a "quotation/estimate" showing the agency/client the itemized breakdown of fees.
If this is the case, I think the Pricing & Negotiating at A Photo Editor might help you give you an idea of formatting your quotation/estimate.
Another option is to find (hire) someone called a "photographer's consultant" in your local area who specializes in preparing bids on behalf of the photographer.
In other words, the photographer's consultant will charge you a fee and the consultant will prepare and write-up the proposal on your behalf. Basically, the consultant negotiates on your behalf.
Try contacting Suzanne Sease Productions to see if she would consider taking you as a client or ask her if she could refer you to someone in your country.