There is no price list for a custom request, because every request is different. Each booster or coach reads your description and estimates the effort: how many games or hours it will take, how demanding the goal is at your rank and server, and whether it needs a particular role or champion. Their offer states a price and an estimated completion time. You compare several offers and choose the one that fits your budget and schedule.
Rank matters most. League of Legends has ten tiers, from Iron, Bronze, Silver, Gold, Platinum, Emerald and Diamond with four divisions each up to Master, Grandmaster and Challenger, and every division holds 100 LP. A goal that is easy in Silver can be hard work in Diamond, so an offer for the same task can vary a lot between accounts. How much LP your account gains per win also changes the number of games needed, which is why it is worth mentioning.
The clearer the description, the more accurate the offers. A good request states the starting point, the exact goal, what counts as done, any limits on champions or roles, whether you want to play yourself or have the games played for you, and when you need it finished. Vague requests get cautious, higher offers or questions in the chat first. Precise ones get sharp prices from players who know exactly what they are signing up for.
Some requests are best split into steps. If your goal is large, such as a long climb combined with coaching, you can ask boosters to price each part separately and decide which parts to order now. You can also discuss the details in the chat before you accept anything, and adjust the request if the first offers show that it can be done differently. Nothing is paid until you have accepted an offer you are happy with.