For landlords, a letting agent is an integral part of the renting process. They carry out lots of the hard work for you, and they also make regular, direct contact with your tenants. They need to be someone you trust completely to do this, without costing too much money.
Whilst your letting agent does, of course, have to be cost-effective, it is also probably not wise to use the cheapest letting agent out there. It’s likely they might cut corners and not conduct the letting process as professionally as you might like.
It is worth looking at the properties a letting agent currently looks after, to understand whether they have experience in the fields you require. Talk to them in depth to understand what they can do for you and what they currently do for others. You can then negotiate rates and come to an agreement on prices that works for you.