There are basically two different ways of how the music budget of a production is being coordinated.
1.) The composer gets a fee for writing the music and any additional expenses for the production of the music (including orchestra recordings, orchestration, copying, mixing etc.) is being dealt with by the production itself.
2.) The composer gets handed the entire music budget the production has and has to pay all the costs from it and is expected to deliver a final recording at the end of the processes without the production handling any of these costs.
The second case is becoming more and more common and is called a Package Deal. There are only few advantages in this. One being the fact that if you work economically you can in the end have more money left than expected.
However, the disadvantages of this procedure outweigh the advantages. It basically puts all the financial risks on your shoulders and can even cause financial loss in worst case scenarios. The only security you can build into such a deal is by negotiating clauses into the contract that limit the risk. This includes limiting the amount of music to be written/recorded, limit the amount of revisions, limit the amount of musicians to be booked, limit the influence of the production in the recording sessions etc.
All these can theoretically save you from needing to spend more than you predicted but negotiating such things into the contract can also feel very suspicious which is not the greatest base in a trusting working relationship. Also the other side will most likely negotiate clauses like “to be recorded with at least x musicians” into the contract to prevent you from doing it all “in the can” when you’re being expected to deliver a real orchestra recording.
The most important thing is to do proper research and calculations of the costs so you are not suprised by any unforseen expenses. If during the process you see that the production side is overstretching of what was agreed on, it is better to address this in a more informal way than throwing any contract clause at them. In the end, most clients are interested in getting a good end product and not ripping you off. Unfortunately though, there are a few black sheep in the industry. So as usual trust your feeling about whether something feels right or not.