Normally, to create a Revit add-in (not too big), I Spent from 5 to 15 working days. With your request is to create more than 1 add-in I think 25 days and 1222$ is reasonable numbers.
Relevant Skills and Experience
Skill: Revit Advance level, C#, .Net
Developed Addins: Autojoin element, Family Browser, Rebar tools, Clash check tools, Model QA tools, ...
Proposed Milestones
$22 USD - Receive the request, evaluate the feasibility
$1200 USD - create Revit addins
If possible, please let me know your idea about addins you want to create.