For small, transactional dealings where the amount of work is easy to predict, there may just be a fixed fee for that.
For more open-ended work, it is often billed by the hour (time and expense). If the work is non-trivial, they may ask for a retainer, which is a down payment against future hourly work and expenses incurred by the firm.
Another common billing model, called contingency, is generally reserved for cases where the firm is optimistic they will be able to receive a significant monetary judgment or settlement, which they will take part of for their time and effort.
For more open-ended work, it is often billed by the hour (time and expense). If the work is non-trivial, they may ask for a retainer, which is a down payment against future hourly work and expenses incurred by the firm.
Another common billing model, called contingency, is generally reserved for cases where the firm is optimistic they will be able to receive a significant monetary judgment or settlement, which they will take part of for their time and effort.