If you are not used to working with freelance writers, there are a few things you should probably be aware of.
First of all, the amount of money that a project will cost may seem higher than you were expecting; this is perfectly normal.
Freelance writers will rarely be in work constantly and do not enjoy the benefits afforded to salaried staff, so their prices will usually have to cover periods of unemployment and underemployment between contracts, together with pension payments, professional insurances and contingency funds to cover illness or other circumstances which would usually be covered by employment benefits.
Costs can often be reduced for longer contracts or those where company benefits are extended to freelance staff. Never be afraid to ask whether they would be willing to drop their rates in light of such considerations.
Secondly, writers will usually ask for their contracts to include payment terms which dictate when and how they should be paid. This is not a sign that they do not trust you, simply a reflection of how much a late or incomplete payment can impact their finances. We do trust you to pay on time and in full, but as the saying goes, it is better to be safe than sorry.
The Writers’ Guild of Great Britain has released a number of guides for writers and those who work with writers, which may be of use. At the time of writing they are available from this link. Video games are included at the bottom of the page.