It is incredibly important for you to source out the cheapest and the best tuition you can. Science or arts, you can’t just wing it and study on your own. You need a tutor to mark as Cambridge would and give feedback.
As for my personal experience with tuitions, I went for SINDA’s A level programme called Saadhana ($50/subject per month). The Math tutor was amazing. The Physics tutor was okay. Chemistry tutor was too fast for me. But I stuck with it from beginning to end because it was affordable, tutors were available to answer questions over the phone 24/7, and I made friends with the students so I could clear doubts anytime. From May to august I added on another Chem tuition (Chemstar Education in Jurong East, $320 per month). After that, I let go of the Chem tuition ( I did not have money to carry on two tuitions simultaneously) and got a private physics tutor instead. This guy was undergrad from NTU and I stuck with him from September to November. The craziest tuition experience I had was for GP. I went to Spring International School (also in Jurong East) which is actually tuition for PRCs who wished to take A levels in SG. The teacher had no experience teaching A levels so I was only charged $120 per month.
All in all, I spent close to $4k-$5k on retaking. I worked part time selling pizza during the weekends to help out my parents a little. My siblings also pitched in to help with my tuition. That is simply the ugly truth of retaking, you can be as determined and disciplined as you want, but you also need money to go on.