Technically you are right, everything has a price. Except when that price is way too high. So if you think a share is worth between say, $5 and $10, they can either say (nicely) "we're not selling shares right now", or say (rightly and rudely) "we can only sell YOU shares for $1000/share". And that may really be a fair price from their point of view, since having you (a nobody in the VC world) as an investor is really detrimental to their image.