How expensive would spices have been in the Dutch republic in 1650? Would middle-class people have had access to say, cinnamon, tea or nutmeg, or were these only accessible to very rich people?

Dutch historian here. Around 1650 the price of, for example, pepper on an auction place in Amsterdam was around 0.47 florin ('guldens') per pound. In 1666 this inflated to 1.10 per pound. As for nutmeg, the cost per pound in Amsterdam was 6.36 florin. In 1666, this doubled and became 12.03 florin p.p.

In theory these spices were accessible for every free man who could pay the price. For example, a Dutch sailor in 'De Gouden Eeuw', or the golden age of the Dutch republic (1602-1702) earned a salary between 10 and 20 florin per month (probably 1500 euros nowadays). I say between 10 or 20 florin, because when a sailor was working in a period of war he would earn more money.

A sailor could definitely purchase some of the more cheaper spices like pepper, but if he wanted to buy a pound of nutmeg that would cost him half a month of salary.

Source: 'Maten, Waarden & Kosten in de 17e eeuw' by Dr. Dirk J. Barreveld.

(You can search for the article yourself, but the PDF-file is in Dutch)

EDIT: Corrected some minor spelling errors