I think it depends on how many days and what kind of places you wish to visit. For Singapore, I think one or two days is enough including Sentosa. For Malaysia, Genting, KL are main destinations, you probably buy the low cost air ticket from Thailand to Singapore and then to Malaysia, there are some cheap tour packages out there also. Check out on the low cost fare then

, ie Air Asia sometimes they provide super cheap rate but beware of the terms and conditions that it is not refundable