In the past I have used the AEM exhaust backpressure sensor kit, which works well. It is about $250 (AEM 30-2064), and includes one of their stainless steel 100psi sensors, as well as a few parts to help cool the exhaust charge before it gets to the sensor. You can also build up the same parts for a bit less cost. I ordered the following parts from McMaster-Carr to make up my own kit:

2x 5482K75 Precision AN 37 Degree Flared Tube Fitting, 304 Stainless Steel Nut for 1/4″ Tube $3.50 each.
2x 5482K82 Precision AN 37 Degree Flared Tube Fitting, 304 Stainless Steel Sleeve for 1/4″ Tube $1.40 each.
1x 5482K111 Precision AN 37 Degree Flared Tube Fitting, 304 Stainless Steel Adapter for 1/4″ Tube OD X 1/8 NPT Male $9.76 each
1x 5482K32 Precision AN 37 Degree Flared Tube Fitting, 304 Stainless Steel Straight Connector for 1/4″ Tube OD $11.02 each
1x 50715K171 Type 316 Stainless Steel 37 Degree Flared Tube Fitting, Straight Adapter for 1/4″ Tube OD X 1/8 NPT Female $9.18 each
1x 4468K811 HT-Pressure Braided Chemical Hose with Fittings, Brass 7/16″-20 Flare UNF Female, 3/8″ OD, 12″ LG $15.49 each.
1x 89895K722 Smooth-Bore Seamless 304 Stainless Steel Tubing, 1/4″ OD, 0.02″ Wall Thickness, 3 ft. Long $18.33 each.

Total cost for parts for a single sensor setup (not including the sensor) was $73.58, plus about $8 shipping.

For a sensor you could purchase the AEM sensor for $152 for a total kit cost of $234, just a tad cheaper than the $250 AEM kit.

However I am using a Honeywell Stainless steel sensor (PX2AN2XX100PAAAX) that is almost the exact same design, but $80 for a total cost of $163. The Honeywell Stainless steel sensors are a great replacement for the AEM ones, in part because AEM doesn’t make the sensor, they just rebrand it from someone like Honeywell. You can get the Honeywell sensor in the same 3 pin Metripack connector, or the more robust Deutsch DTM.

You could also make a cheaper kit by not using all Stainless steel parts. The AEM kit for instance doesn’t use a stainless steel -4 to -4 male connector, and one of the two NPT parts isn’t stainless steel. I prefered to use all stainless parts to prevent corrosion in a part with very high heat and exposure to road water.