25(OH)D test from Grassrootshealth is the cheapest source I knowat $40
and Rosebud is quite right it is the test to measure the circulating level of 25 hydroxy vitamin D.
When your doctor gets the result you need to understand the goal posts have moved over the last few years. This Mercola article spells it out.
To get into the 50~65ng/ml range really is going to require roughly 5000~6000iu/daily and you may need to take 50,000iu ONCE A WEEK
for probably 8 weeks to get status up ASP. (some might suggest one 50,000iu every 5 days to speed up to target faster)
You do need to check if your doctor is going to prescribe Vitamin D that the correct effective long lasting human form is going to be supplied. Some pharmacies are putting a label on 50,000iu ERGOCALCIFEROL capsules that implies they are Drisdol ~ MAXIMUM D3
however MAXIMUM D3 supply only 10,000iu d3 CAPLETS while the DRISDOL are ERGOCALCIFEROL and are actually D2. I have had many online arguments with people who think they have been prescribed D3 and are taking 50,000IU and it looks like this
and they find it hard to accept the truth that this is in fact D2 ergocalciferol.
D2 AND D3 are NOT the same thing.
As the price of D3 is so cheap and initially you will use one ever 5~7days then probably when status reaches 60ng/ml 150nmol/l drop down to perhaps one x 50,000iu every 10 days = 5000iu/d it's hardly worth getting them on prescription. Even if you moved over to 5000iu
the cost remains so trivial it's hardly worth the trouble going through your doctor.
PS The suggested ranges above of 65~90ng/ml are to support Cancer Treatment. They are developing patented analogues of vitamin D3 that they claim will be 10X more effective than the natural form (and inevitably a lot more expensive) so until these are available using the natural form in larger amounts as Dr Cannell sets out here
is probably going to be helpful.