The output of your work is worth more than what the company pays you. That is how a company stays in business, by outputting higher value than the cost of production. So by this rationale, the real cost for the company during your vacation, when you're not producing anything, is not proportional to your salary. From the company's bottom line perspective, one extra vacation day is not equivalent with 1/200 of your annual salary, it's more.
That said, there is no general value for "reasonable" here. The company will certainly think it unreasonable if you ask for significantly more than is the local industry norm where you are. And again, since not having your employees around is a huge inconvenience for the company, be prepared to pay a premium for it. Be also prepared to commit to taking any extra days you negotiate during a fixed period of the year (like 3 days extra, but you must plan them in July).
Then finally be prepared for the company being inflexible in this matter, regardless of salary. Especially larger organization tend to want to keep remuneration packages fairly homogenous across peer-groups. This to avoid the "why is he getting X when I'm only getting Y..." discussions.
- Everything is negotiable, but to what extent differs greatly between companies, regions and countries
- Try to be as flexible as possible on your end. For instance, commit to taking some of your days during only certain periods.
- The relationship between dollars and vacation days depends on the value of your output and the opportunity-cost for the company of not having you available for periods of time. If your work is such that it can be predicted and planned long ahead of time and your responsibilities can be shouldered temporarily by someone else in your absence, then it's less expensive to have you leave.