figure out your hourly speed. and divide the length of the railing by that.
you want to sand all rust and scuff the old paint. wipe clean. 1 coat with a rust inhibitor. then 2 coats with oil (tremclad is great, also para does an affordable black oil i've used on many railings and doors with great success)
figure out how long it will take you to do the process on an 8 foot length then divide that into the entire length of the fence. figure out how many days it should take you, and charge what you need for your time plus materials.
it's tedious time consuming work. goes faster with a helper on the other side of the railing to catch your drips on the other side and vice versa.
cheers and good luck,
j