The fixed price is give based on several factors:

Clarity of the scope
The simple rule is that the clearer the scope, the easier it is for us to estimate a timeline and hence the quote is more precise. On the other hand, if the scope that you are providing is very open then we have to add buffer in the pricing to make up for any surprises.

Size of the project
Larger size projects tend to attract better pricing as they ensure cash-flow for a larger period of time. Shorter project on the other hand gets quoted on a higher end.

Skill level and experience of project team
For complex projects or those with a shorter timeline we need to allocate better skilled and experienced resources. People with higher experience, knowledge and better skills are paid higher and hence the hourly rate for a project depends on that. Under most cases, the hourly rates vary between $12 and $15 per hour.

Lead time to start a project
Lead time is defined as the duration between formally committing to a project (i.e. signing contract and sending advance) and the day when the project officially begins. If the lead time is short, then we have to do more dynamic allocation of resources across multiple projects and there is a lot of overhead. Hence, such projects get quoted at a premium fees.