well muggs you can spend about any amount of money on remodeling as you would like. just depends on how extravagant you would like to be. the sink or shower into your sump pump is a big no no. those pits are not sealed like a lift station/ejector pit for sewage would be and it will get to stinkin even if its just grey water. if your sewer line is under your floor and relatively close to where you want to put your bathroom it will save you some money. remember you will need to have one eigth to one quarter inch per foot fall on a sewer line so if the line under your floor is on the other side of the house you made need a lift station after all. i think someone said 1500 bucks before, thats probably gonna be close for a rough in and gettin your floor poured back in.
as for the taxes around here if its finished its taxable, but you can get around that here by not finishing the cielings and painting them white. not the best look but it saves a few bucks. but it will count as another working bathroom and that will raise the taxes. if you are a fairly handy guy you could save some money doing some of the labor yourself such as breaking the floor out and getting rid of it and mixing and pour the cement back.