When new, 530 was easily RM100k more than 520. now even cheaper than 520 when used. bearing in mind that the maintenance cost is similar, the only "extra cost" would be the once a year road tax which should not be a concern for Beemer owners who always yearn for "more power" and even try the extent to remap or supercharge the engine.
let take a practical scenario
used 530 RM53,800 vs 520 RM59,000
if you bought a used 530 and intend to keep for 5 years, your "extra cost" would be around RM1,500 per year (road tax) x 5 = RM7,500, less of initial cost savings RM5,200 bal = RM2,300
To enjoy drastically more naturally aspirated power over 5 years @ merely RM2,300 in total is too good a bargain to miss.