If your budget is tight, my recommendation is to first look for a used 1080ti. It should provide a fairly good experience @1080p. Stay away from cards that have been used for 24/7 mining.
Next you could also get two more RAM sticks for 32gb total if your MB supports it.
Next is CPU overclocking: you could look into a beefed up AIO cooler: with something like a H100i you could shoot for 4.8Ghz with 1.296V (1.3V is the max recommended for haswell/haswell refresh). Best is to research Overclocking forums for advice on the optimal combo.
AFAIK your current MB Z97P is not intended for max overclocking potential. You may need to look for a used motherboard for a few bucks to push your CPU to its limit, in order to mitigate main thread bound scenarios in MSFS.
Finally, MSFS needs to be on SSD or better NVMe if it’s not the case already.
Depending on where you are located, prices can vary greatly.
A tentative budget looks like this :
New SSD 512gb = 50 dollars
1080ti, used = 640 dollars
Two DDR3 sticks, used = 80 dollars
Overclocking MB, used = 80 dollars
CPU Cooler, used = 50 dollars
Total 900 dollars (maybe more, these days GPU prices are difficult to predict)
Not cheap, for someone on a budget, but hopefully enough to enjoy MSFS at 1080p with fairly high settings yet cheaper than a brand new 12900k/3080 combo at USD4000++ 
Any other significant upgrades involve a new chipset, new MB, new CPU, new cooler, new RAM sticks.
Another even cheaper option is to get an Xbox S/X series, I have not tried it but reviewers are saying it provides good experience when it works
However you would loose the benefit of third party extensions, etc…