I feel like EP's are overpriced in sims 4. They cost the same as sims 3 EP's (40€) and offer less gameplay features. As someone mentioned above it also depends on interest. If the theme of a pack really matches my personal interest, I can be persuaded to buy it at full price. Otherwise I will wait for sales or bundle them.
The price of a GP (20€) is reasonable to me. When I first heard about them I expected them to be would-be EP's but they offer enough interesting gameplay for a good price. I even like some GP's more than the EP's that don't suit my gameplay, which is something I did not expect when I bought my first GP.
SP's feel like a money grab. It is nice to have some gameplay in them but the only one I ever bought full price was toddler stuff. I don't regret it as this is one of my favourites. But most SP's have to little impact on my game to pay full price. Only exception is laundry stuff. This pack is not completely my cup of tea but this one seems to offer more gameplay than most SP's.
As for sale, they seem to come round regularly for EP's and GP's. unless when I am waiting for one like right now. ;) SP's don't get much sales which is a shame because they need it just as much to sell.
The only thing which bothers me about sales is if they only appear in specific countries. It is a real disappointment to read online about a sale on origin where all EP's get a discount of 50%, just to find out it is only 30 % for people who don't live in the USA. This was the case for C&D for example. This only happens for the first sale the latest EP, or in other words the most interesting one to come on sale.