Do they actually need to be of that class, or is that an example?
If you're considering treating them simply as hirelings with one job (i.e. the herald isn't supposed to fight or cast spells, just announce you when appropriate), you can also look at simply hiring them based on their level, as if they had a maxed out Profession skill appropriate to their level and took 10.
So, if you have a 5th level Profession: Herald, then you're looking at a 9+ GP per week (18 average check with no stat modifier). That should be a very base for skilled hirelings.