Actually, your upvote value did not decrease.
I understand that there might have been a slight decrease in your upvote value due to the fluctuations in the price of STEEM. Steem Power (SP), after all, is nothing more than STEEM that has been staked into the Steem Blockchain. So, with the fluctuations in STEEM Price your upvote value is bound to fluctuate, as well. However, I don't see this to be the reason for your "decreasing upvote value".
It's entirely possible that you might have mistaken your upvote value when you had 100 SP. The thing is, at 500 SP, your upvote value is just right at $0.025 (rounded off to $0.02). Perhaps you remembered your upvote values incorrectly back when you had 100 SP, or maybe you had an incoming delegation back then that increased your effective SP and, as a consequence, also increased your upvote value, but the upvote value of a 100 SP account is actually just $0.005 (rounded off to $0.00).
In my case, for example, right now I have just over 105 SP and my upvote value is at $0.005. (I actually have a higher SP, but due to outgoing delegations, that's my effective SP at the moment.) So, your upvote didn't really decrease. It's just right for your SP.
If you want to compare different levels of SP and their corresponding upvote values, you can go to SteemPeak (https://steempeak.com/@akdx) and look at the wallets of different users. They display up to three decimal places, so it's more accurate when you are looking at smaller SP increments in the hundreds. You can view the vote values by hovering your mouse at the "i" Info button on the right corner above the account's profile description. Just compare that with the effective SP in the user's wallet.
You can also check out SteemNow (https://steemnow.com/upvotecalc.html). It's actually a vote value calculator, but the downside is it only shows up to two decimal places.
Hope this helps.
The question can be answered very simply by saying the price of Steem has dropped.
Your account has grown but the Steem price dropped reflecting your current vote. if you hadn't of grown your 2c vote would now be 0.004. You personally have grown the account by 500% but Steem has dropped by the equivalent keeping your vote at exactly the same point.
It will grow in the future when steem bounces back and if it gets back to the same point will be worth 10 C as you have grown 5 x. Don't worry about it as volatility is what crypto is all about for now. Just grow and don't worry it is the same for everybody.
I've actually written a blog post all about this: "What controls the value of our payouts?".
But the gist of the said post is that, there are at least 4 factors that determines the value of a vote: rshares, reward_balance, recent_claims and the sbd_median_price.
upvote_value = rshares / recent_claims * reward_balance * sbd_median_price
Using your question as reference which was upvoted by @musing and two others, and SteemBlockExplorer for all data numbers, the upvote value is calculated as:
upvote_value = 371011592608(rshares) / 519,212,470,000,000,000(
recent_claims) * 766,671(
reward_balance) = 0.54783705159 STEEM
Right now 1 SBD = 0.79 Steem so: 0.54783705159 STEEM *0.79 = 0.4327912707561 upvote worth which is the upvote you got above.
My point here is that the value of an upvote is heavily influenced by the price of SBD and Steem in the market.
If the price of SBD and Steem is only around $2.5 then you would have gotten an upvote of: 0.54783705159 *2.5 = 1.37 here!!! instead of 0.43.
That is why everybody's upvote value is decreasing, mostly because of the disappointing price lately
Steem Power increases at an annual interest rate of about 5%.
It's the only guaranteed form of ROI Steemit provides.
You can literally just HODL your vests of STEEM and make money, assuming that the STEEM price don't drop between now and then.
However, the vote value is tied directly to the reward pool and number of vests being engaged in Up-voting.
Cannot provide you a formula for vote weight, I don't dig deep in the technical ends.
The most important thing to take in is this.
Reward Pool is finite.
You can only get a cut, proportional to the Steem Power you hold in relation to all Steem Power being engaged on the whole platform in that week.
The more Spam and other forms of Abuse gets up-voted and goes un-flagged until it reaches payout, the less SBD/SP there is in the Pool for everyone else that week.
Recently, we've been witnessing an increase in draining of the reward pool.
The vote value have been dropping significantly.
There are few reasons for why it could be happening.
However, I will not be exploring this any further at this time.
The point of your question wasn't related to that.
That's because the STEEM price is difference few months back, if the STEEM price is back into few months' value, your 500SP will worth maybe 0.08 for example. Don't be disappointed, it will be back on it's course towards the moon and you'll be thrill that your upvote will be more by the time.
Steem power is the amount f stake that you hold on this blockchain. Vote values are basically based on the internal market evaluation. If you were there in the first quarter of this year on steemit then you might have found the difference between the vote value. During the rise period my 20 sp account upvoted people at a value of 0.1 but as of today I stand at 100+ sp and my vote is considered as dust.