The Collector's Trove wrote in TSR Printing Records:Originally Dave and Gary each got "a buck" for every copy of D&D sold! That's 20% of the sale price! This was the same for Warriors of Mars (Blume 5%, Gygax 15%)
increment wrote in TSR Printing Records:My understanding is that under the original contract, they got a buck for every copy of D&D that TSR sold itself retail at full price - that is, through direct mail order or in-person sales (all pre-DHS of course). The issue was, retail was a small percentage of the copies they sold. Many were sold wholesale or job lot at below 50% of cover price. So in effect, the 1975 revision (which gave 10% to the cover price instead) to the contract wasn't halving anyone's take - it broke even if virtually all sales were at 50%, and given how many sold below 50% at the time, could even be more advantageous.