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You should go with monthly bases or project bases, Let's find out quickly
You have a project, Which is ongoing. Ex: Suppose you have 5-10 things to do for now, Then more 5-10 for next version and so and so. You must go with monthly bases. Also There is many companies out there which is taking our white label services to deliver their client projects on time.
When you have fixed list of things to work on, which will not going to change before project delivery. Then and then only you should go for Project Bases. Else We would definitely suggests you to go for Monthly Bases. EX: if you have 10 things to work on, And before delivery you thing to remove few things and add up new 4-5 things, It will change delivery time and cost.