The most common charge you'll be quoted from firms is 5% of monthly rent to manage, and one month rent to rent the unit.
You'll find a large variance from firm to firm in fees charged, and how the charges are structured.
So for example one company might quote $90 per month flat fee instead of 5% of gross, another company might quote $1100 charge to rent vs one month of rent.
You'll also find discounter type firms - with re-rentals starting from as low as $400 per unit, etc..quality of service, scope of work offered, and other factors (lack of insurance, skill set, etc...) will often account for this.
You'll find ones that offer unlimited service for the $X per month, others will claim this and then quit or asking for additional fees when too much work is generated, others will charge $XX per month per visit to property needed if in excess of X amounts, and others will charge a percent mark up on invoices or an extra amount of $XX per maintenance event IN ADDITION to the fee you're charged from workers. All of this should be set out in your contract.
Other firms, and I do this often, will not charge a rental fee but charge a higher rate for property management and include renting as a free service - so for example at 11% of gross - but this will vary materially from property to property
Property management is a low bill rate service that has a huge impact on your bottom line over time - I'd strongly recommend you choose a property management company based on service, not price. As an owner of a property management company I know that if I was ever investing in a different area I'd be more likely to choose the costliest company offering services in that area than the cheapest. When you hire a company for a job that requires 3 hours a month of work and they only charge you $50 for it what unfortunately often happens is you end up with less than the needed amount of work being done each month.