Obviously- attached is a lot nicer and more convenient.
You could also do a carport for less money. Though some people don't like those.
If you want someone good and local to give you a quote, please let me know. I've worked with a few people. Thanks, and good luck,
I believe 30K (if you do not do any of the work) is a good estimate.
Are you looking at a home with no garage now? If so, make sure there is enough room to add one. Check zoning laws in the location and also check out what is under the ground in the anicipated build area.