Portion = a share of money or other property allocated to an individual, often, but not necessarily, in a will or family trust. Dowry = a marriage portion, often, but not necessarily, a sum of money. The terms on which the portion was transferred depended entirely on the terms of the transferring document. It was always possible, but not normal, to "protect" the position of a bride by transferring the portion to trustees for her, so that her future husband would receive only the the interest on it and would get nothing if he became a childless widower. This was a matter of pre-nuptial negotiation.
Price indices have been produced (see eg measuringworth.com) and they work fairly well for working-class families that spent most of their income on food. But for anyone with £1000 it is difficult to compare values over a period as long as a century, because purchasing power can only be measured by what is likely to be bought, which would not have been the same. English society (except for the working classes) was richer than in 1810 than it was 1695. IMy guess is that in the context of the marriage market £1000 in 1695 might equate to £3000 in 1810 - less than half what Maria Ward had to attract Sir Thomas Bertram [Mansfield Park].
