Answers

Always seal first and then paint. Why are you re-sealing the corners? 

Seal the corners first, prime, and lastly paint.

Seal first, repaint second, sealant is like caulking used to cover openings.

That is going to vary depending on where you live, the particulars of your home construction, as well as particulars of your planned "pop-top".  Here in Richmond, VA, the structure and exteriors, plus a window or two for a relatively narrow dormer runs in the $10-12,000 range... before you make interior renovations to the bathroom which presumably you may be seeking.

I am guessing that you may be interested in something wide than a 3-4 foot typical dormer.  Well, things become much more structurally complicated in a Cape roof, the wider this pop-top becomes.  As the roof loads become asymmetrical, beams and opposing side structural work will likely become involved (ie: you may have to add structure on the OTHER side of the house, and a beam at the peak of the roof... possibly posting-down to the foundation, to balance the load).  I have seen those go to $30,000+... again, before you start adding in the bath renovations below.  I know; sounds shocking but in order to maintain the structural integrity of your roof, these things may be neccessary.

A good way to add value to your house is to do home improvement. Many renovation and home additions add value to the house. Also, painting by interior painting San Jose is a good way to add value to the house. One of the simplest and most cost-effective improvement is house painting. Low-maintenance landscaping also adds value when you sell your house.

Adding the stylish furniture in home can be very effective in increasing the home vlaue. 

You might want to look into AB2299 for the state of California. Add additional living area and raise the value 160k-400K to all R1 Properties. We build these for a living and are seeing a 2% return rate on your investment.

Direct added value to your home:

Finish out unfinished living areas

Upgrades to bathroom and kitchen

Add additional living area

All other items completed to home have negligible value added benifits in terms of Apprasial added value.

Same as others said, Remodeling magazine's Cost Vs. Value report is a great resource!

As a roofing contractor, of course I am going to suggest an upgrade to your roofing system. And that's not just a biased opinion either. Working with a lot of realtors in our area it has been proven that the roof can be as much as 40% of your curb appeal. Especially when the system incudes things like the right color drip edge on the rakes and eaves, and high profile ridge caps. Both of those items are relatively inexpensive and can make a very noticibale difference compared to homes that don't have them. Selecting the proper type of shingle of course also plays a huge role. About 95% of re-roofs these days use a standard architectural tpye compositions shingle but picking something from the designer line can realy make your home stand apart. Then again, other rof covering options besides ashphalt shingles can make an even greater impact. 

As a GAF Master Elite Copntractor, we are partial to their line of products and I have included a link below to their designer apshpalt series.

Good luck!

http://www.gaf.com/Roofing/Residential/Products/Shingles/Designer

I agree with Chris.  The Cost vs Value report is the "go to" guide and is impartial.  Personally, I like to focus on first impressions and curb appeal.  Exterior dressings like corwn mouldings and simple shrub pruning is a great low cost way to give the home a wow factor.

You might also contact a couple of local realtors to see what people in your area are looking for in the home they want to purchase.