Here are points to consider:

1. Cannot be a contingent fee arrangement. This is unethical and would be
disastrous regardless… enough said.

2. Cost of initial consultation. This should be free or nominal, but make sure this
is clear.

3. Definition of scope of work. This should be set out in the initial engagement
letter in a straightforward, succinct manner.

 
4. Determining whether the case will be billed hourly or in stages. This should
be discussed at the front. Are you doing it by the hour or by the job to be performed? By the job
avoids runaway expenses, but can lead to experts cutting short the work that needs to be done.
5. Setting a budget. This avoids surprises for both sides and eliminates the stress of
the unknown.  

6. Regularity of billing statements. Same as above. It eliminates stresses and
surprise by not including the expert’s bill in your final statement of charges or in the
reconciliation to your client.  

7. Estimate of costs associated with forensic testing and/or scene work. Such
work can involve outside contractors or specialists. You want to budget these out as well.

8. Cost benefit analysis of economy versus completeness. If you cannot afford to
do everything necessary, cover it with your client first! Explain that the costs ultimately are
either paid directly or indirectly. Some clients will raise hell after you settle their case even  though you may have fronted the expenses and resolved the case very favorably. Keep your  client informed of the cost in advance of incurring it.  

9. Cost associated with satisfying federal court or state court disclosure
requirements. What costs in your jurisdiction are to be borne by the party, versus the opponent?
Initial disclosures or answers to interrogatories are usually the financial responsibility of the  party who hired the expert.  

10. Cost associated with responding to discovery requests. Additional requests for
information or discovery may not be had for free. Under Federal Rule of Civil Procedure
26(4)(E):  

Unless manifest injustice would result, the court must require that the party
seeking discovery:  

(i) pay the expert a reasonable fee for time spent in responding to discovery under
Rule 26(b)(4)(A) or (D); and  (ii) for discovery under (D), also pay the other party a fair portion of the fees and  expenses it reasonably incurred in obtaining the expert’s facts and opinions. 

 If you are in state court, check your jurisdiction’s law and make sure your expert is paid
in advance of doing the work.  

I hope you strike the right deal.