For the budget you provide, the only ISOFIX carseats available will only allow use of the ISOFIX for front facing. You'd have to still use the seatbelt for rear facing. Examples are Koopers Salsa and Santa Barbara ISOFIX carseat, which also rear face only to 13kg.

If I'm not mistaken, the cheapest rear-facing ISOFIX carseat is Koopers Jive, which rear faces only to 13kg too, but is RM879 at littlewhiz.com. I'm also unsure of its suitability to your vehicles. Do check directly with the shop/distributor. ISOFIX tends to be rigid in whether it suits your car or it doesn't. There is no adjustability with ISOFIX.

If your budget cannot stretch past RM800, you may want to look at LATCH, instead of ISOFIX. LATCH is the USA version. It's cheaper than ISOFIX because it's essentially a belt (made of the same material as a car's seatbelt) provided with the carseat that you then connect to your car's ISOFIX points. LATCH carseats that can be within your budget on lazada and are carseat technician recommended include Cosco Scenera Next, Cosco Apt 50 and Evenflo SureRide. Lazada prices change every day though, so the prices can go beyond your budget and different sellers price differently too. These carseats all rear-face beyond 13kg (rear-facing is SAFEST; therefore rear face as long as possible), and suits most cars with ISOFIX attachments, but LATCH can be harder to install than ISOFIX, yet because of the adjustability of belts, they are more flexible to suit more cars than ISOFIX. Buying online via lazada does mean that you can't be sure what manufacturing dates you get, and that could mean shorter lifespan of carseats (because USA carseats have fixed expiry dates).