I'm just coming through dealing with this myself, and believe me I know how tough it is! I couldn't get off the couch and was completely useless in the house. For what it's worth, it's not uncommon, and it's not dangerous, but it sure is miserable. I haven't had mine measured in this pregnancy yet, but in my last pregnancy, which didn't make me feel as lousy as this time, it was about 80/50 in the early months.
Drink as much as you can, and eat salty stuff, along with good protein. If you can manage it (and if you're like me, you won't be able to for a little while yet...) try a bit of exercise, just to get your blood moving. Also, do you think your iron might be low? Low blood pressure and low iron can go together. You could maybe have some bloodwork done to check, or you could just take some supplemental iron. About 50% of pregnant women do have low iron. I increased my iron intake and started feeling better pretty quickly, although it could also be related to how far along in the pregnancy I am now.
Hang in there though - it should start to improve soon!
All the best!