How much does a repeat hair transplant cost, approximately?

A repeat hair transplant cost as much as the first if you're doubling the work. The cost depends on the number of grafts as well as the technique in harvesting the grafts.  

Yes, it is definitely possible to enhance your existing hair density by undergoing another hair transplant procedure. There are actually two methods that are available. It sounds like you have already undergone a strip surgery. But a newer technique is known as Follicular Unit Extraction which does not leave a linear scar. The grafts are extracted directly from the donor surface using tiny punches, not a scalpel.Procedures are typically priced by the graft. Costs range from about $5-$12 a graft depending on the clinic. And the number of grafts you would need actually depend on a number of factors such as: the existing density of the surrounding areas, hair texture and hair thickness. Fewer grafts are needed for curly and or thicker hair shafts. 

The best solution for thin hair after a hair transplant is to increase the density of hair through another hair transplant procedure.  It is perfectly okay to have more than one procedure to add to the initial density.  One hair transplant usually does not create more than 40% density. It is usually sufficient to eliminate the balding look but it takes more to have a perfectly solid hairline.  A repeat hair transplant can add hair in between the already transplanted area and is usually a very successful procedure. If you had a strip procedure you can have another strip that usually leaves you only one scar (it removes the old scar and leave a new scar in its place).  You may also decide to go for a FUE that extracts hair grafts from the neighboring areas of your donor zone.

Absolutely a repeat hair transplant is possible to increase the density.   In fact, many people require a second transplant down the road if the original hair is lost (the transplanted hair of course is permanent!!)It sounds like you had  strip procedure 10 years ago. The second procedure could also be a strip (or you could have FUE) but there is no advantage in this case to FUE over strip. The hairs would come from the same area at the back of the scalp. Most surgeons would take out the original strip and stitch it up again so that you'd only have one scar (not two).How many hairs do you need?Impossible to say without a photo - but in general a 'thin' area that is 1 inch x 5 inches needs 500-900 hairs. Prices vary around the world, but in North America fees would range from $ 2, 500- $ 8,000

Yes, you can increase hair density with a second hair transplant. Many people in our practice pick the alternative which is scalp micropigmentation instead of a hair transplant for increasing the look of fullness which is a 'look' not a number issue. See examples below in the web-reference

The cost depends on many factors which includes the size of the area where you like to add density and the number of grafts, the Experience of the doctor and technique recommended. It is best to do a consult with a few doctors to get a general idea about the number of grafts needed and get an idea about the cost.