Best underlayment for Engineered hardwood on concrete

jcieutat

I purchased around 1250 square feet of 1/2" x 5" Handscraped Engineered Hardwood today. The salesman was trying to sell me an underlayment that has an adhesive on both sides. It sounded great but was $1.70 a square foot. That is just too much considering what I just spent on the flooring. Our house has stained concrete floors but we are doing an addition and it just will not match so we are putting wood over it. My slab is not in bad shape but it is certainly not perfectly level either. Can anyone recommend an underlayment that will provide a good moisture barrier and also make it feel like it is solid and not hollow. I also have numerous surface cracks and am wondering if they should be filled with something. I am not looking to go cheap here but I just don't have enough to spend $1.70 a foot. Or should I just have the installer glue it down? Thanks