An affordable lash set is one that suits your lashes, gives you the finish you want and has a maintenance cost you understand. A low price does not by itself prove poor quality, and a high price does not guarantee a good result.
Eyelash extensions need maintenance as natural lashes grow and shed. When to book a refill depends on how much of the set remains, its condition and how full you want it to look.
Eyelash extensions may be worth considering if you want visible lash definition without applying mascara every day. Whether they are worth it for you depends on the look you want, your budget and your willingness to maintain them.